Method for making thin film semiconductor

ABSTRACT

The present invention provides new and improved methods for making crystalline semiconductor thin films which may be bonded to different kinds of substrates. The thin films may be flexible. In accordance with preferred methods, a multi-layer porous structure including two or more porous layers having different porosities is formed in a semiconductor substrate. A semiconductor thin film is grown on the porous structure. Electrodes and/or a desired support substrate may be attached to the grown film. The grown film is separated from the semiconductor substrate along a line of weakness defined in the porous structure. The separated thin film attached to the support substrate may be further processed to provide improved film products, solar panels and light emitting diode devices. These thin film semiconductors are excellent in crystallinity and may be inexpensively produced, thereby enabling production of solar cells and light emitting diodes at lower cost.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-In-Part of commonly assigned U.S.patent application Ser. No. 08/595,382, filed Feb. 1, 1996 now U.S. Pat.No. 5,811,348.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a thin filmsemiconductor, a solar cell, and a light emitting diode. Moreparticularly, it relates to a method for forming a semiconductor filmlayer on a substrate having a plurality of porous layers defined thereinhaving controlled and differing relative porosities.

As the material of the solar cell, various materials have been studied.Silicon, for which there are abundant reserves and which is free fromapprehension of pollution, is the center of these efforts. Ninetypercent or more of the amount of production of solar cells in the worldare silicon solar cells as well. The tasks in solar cells are how toachieve a low cost, a high efficiency of conversion of light toelectricity, a high reliability, and a small number of years for energyrecovery. For the requests for high conversion efficiency and highreliability, single crystal silicon is most suitable, but it isdifficult to fabricate single crystal silicon at a low cost. Therefore,at present, in the field of solar cells, particularly solar cells havinga large surface area, active research and development is proceeding onsolar cells using thin film polycrystalline silicon or thin filmamorphous silicon.

In a thin film polycrystalline Si solar cell, the purity of the siliconis raised by refining techniques from metal class silicon using plasmaor the like. An ingot is prepared by a casting process, and a wafer isprepared by a multi-wire or other high speed slicing technology.However, process for removing the boron and phosphorus from the metalclass silicon, the preparation of an ingot of a good quality crystal bya casting process, enlargement of the surface area of the wafer, and amulti-wire or other high speed slicing technology require a very highgrade of technology, so a substrate which is sufficiently cheap and hasa good quality has not yet been fabricated at present. Further, the filmthickness of the wafer is approximately 200 μm, therefore a flexiblesubstrate cannot be formed.

Amorphous silicon can be formed on the surface of a plastic substrate bya CVD (chemical vapor deposition) process. Therefore, it is possible toform flexible thin film amorphous silicon. As a result, solar cellshaving a wide range of applications can be formed. However, there aredrawbacks in that the conversion efficiency is lower than that of thepolycrystalline silicon and single crystal silicon, and the conversionefficiency deteriorates during use.

Single crystal silicon offers the promise of a high conversionefficiency and a high reliability. Thin film single crystal silicon canbe fabricated by the SOI (Silicon On Insulator) technique, which is amanufacturing technique of integrated circuits etc., but theproductivity is low. Using the SOI technique, the manufacturing costbecomes considerably high, this is a problem in application to a lowcost solar cell. Further, the processing temperature in the preparationof single crystal silicon is relatively high, so it is difficult to formthis on a plastic substrate or glass substrate having a low heatresistance. Since it is difficult to form single crystal silicon on aplastic substrate, the manufacture of flexible thin film single crystalsilicon is difficult.

When constructing window glass equipped with solar cells, in otherwords, solar cells are arranged on a surface of a window glass, solarcars with solar cells arranged on the roof, etc., the use of a flexiblesolar cell is desirable from the viewpoint of the simplification of themanufacture and the ease of rational arrangement for enlarging the lightreceiving surface. Nevertheless, the only semiconductor silicon whichcan be used to make the flexible solar cells at the present time isamorphous silicon.

SUMMARY OF THE INVENTION

The present invention provides a method for making a thin filmsemiconductor with which a thin film semiconductor, for example, thinfilm single crystal silicon, can be reliably produced on a massproduction basis. Therefore, a reduction of cost can be achieved andprocesses for producing a solar cell and a light emitting diode withwhich a solar cell having a high opto-electric conversion efficiency canbe reliably and easily produced at a low cost.

In an embodiment, the present invention provides a process for producinga solar cell with which the terminal of the solar cell can be easily andreliably led outside with a low resistance.

In an embodiment, a new and improved method for making a thin filmsemiconductor is provided comprising the steps of providing asemiconductor substrate having a surface. The surface portion of thesubstrate is treated in at least one anodization process to define aplurality of porous layers having varying degrees of porosity adjacentthe surface. In a preferred embodiment, the substrate is anodized in afirst anodization step at a first current density to provide a firstporous layer adjacent the surface having a first porosity. A secondanodization step is performed at a second, higher current density toprovide a second porous layer adjacent the first porous layer oppositethe surface having a porosity greater than the first porosity. Thedifference in porosity between the first porous layer and the secondporous layer provides an inherent line or zone of relative weaknesslocated in the second porous layer or at or adjacent to the interfacebetween the first porous layer and the second porous layer. The line ofweakness or fragility introduced by the strain caused by the differencein the lattice constants of the adjacent porous layers permitsseparation of the surface layer and any film grown thereon from theremainder of the second porous layer and the substrate. In an especiallypreferred embodiment, a third anodization step at a third higher currentdensity is performed to define third porous layer having a thirdporosity higher then the second porosity. The third porous layer isdisposed within the second porous layer or adjacent to the second porouslayer. In accordance with this embodiment, a relative line of weaknessis defined by the third porous layer or at or adjacent an interfaceformed between the third porous layer and the second porous layer. Aftera plurality of porous layers are defined adjacent the surface of thesubstrate, at least one semiconductor film layer is formed on the firstporous layer and surface. Thereafter, the semiconductor film isseparated from the semiconductor substrate along the line of relativeweakness to provide a thin film semiconductor product.

According to the present invention, a thin film semiconductor isprepared by a changing a surface of a semiconductor substrate to form aporous structure comprising two or more porous layers having differentporosities; growing a semiconductor film on the surface of the porousstructure; and separating, removing and/or peeling the semiconductorfilm from the semiconductor substrate in a controlled or directed manneralong the line of weakness created in the porous structure layers.

Further, in the process for producing a solar cell according to thepresent invention, a solar cell is produced by method comprising thesteps of changing the surface of a semiconductor substrate to form aporous structure comprising two or more porous layers having differentporosities; epitaxially growing a semiconductor film comprising multiplelayers constituting the solar cell on the surface of this porous layer;and peeling or otherwise separating the multi-layer epitaxialsemiconductor film from the semiconductor substrate along a line ofweakness defined in the porous structure layers.

Furthermore, in a process for producing a light emitting diode accordingto the present invention, the light emitting diode is prepared by amethod comprising the steps of changing the surface of a semiconductorsubstrate to form a porous structure layer comprising two or more porouslayers including a first porous layer constituting a light emittingportion and a second porous separation layer, having a higher porosity,disposed beneath the light emitting portion and thereafter, peeling orseparating the light emitting portion from the semiconductor substratealong the line of weakness defined in the separation layer of the porousstructure.

As mentioned above, according to the process of the present invention,the semiconductor substrate surface per se is changed to form the porouslayer, a semiconductor film is formed on the substrate by epitaxialgrowth, and this semiconductor film is peeled from the semiconductorsubstrate by breakage in the porous layer or at an interface with theporous layer, thereby the intended thin film semiconductor or solar cellis obtained. Accordingly, the epitaxially grown semiconductor film canbe formed with any sufficiently thin thickness. Further, the peeling ofthe thin film semiconductor from the substrate can be reliably carriedout by appropriately selecting the strength of the porous layer, forexample, by the selecting the porosity in the porous layer. As describedabove, according to the present invention, a thin film semiconductor canbe obtained with any sufficiently thin thickness with a good yield.Further, in the production of solar cells, a solar cell having asufficiently high optoelectric conversion efficiency can be formed forthe reasons that the active portion to be constituted by this epitaxialfilm can be constituted sufficiently thin and it can be formed by thesingle crystal thin film semiconductor layer, which is epitaxiallygrown. Further, since it is now possible to provide a flexiblestructure, various applications, for example, applications for windowglass equipped with solar cells, solar car, etc. become easier toproduce.

Further, in the light emitting diode embodiment according to the presentinvention, a superlattice structure can be formed by the porous layershaving different porosities and therefore an improvement of the lightemitting efficiency can be achieved.

Other objects and advantages of the present invention will becomeapparent from the following Detailed Description of the PreferredEmbodiments taken in conjunction with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of an example of an anodizingdevice for working the present invention;

FIGS. 2(a)-2(c) are cross-sectional views illustrating the sequence ofsteps for forming a porous structure on a semiconductor substrate inaccordance with an embodiment of the method of the present invention;

FIGS. 3(a)-3(d) are cross-sectional views illustrating the steps ofgrowing an epitaxial semiconductor film, bonding a structural support tothe semiconductor film, separating the semiconductor film from thesemiconductor substrate along a line of weakness defined in the porouslayer structure to provide the separated semiconductor film product;

FIGS. 4(a)-4(d) are cross-sectional views illustrating the steps forforming a second porous structure in accordance with an alternateembodiment;

FIGS. 5(a)-5(b) are cross-sectional views illustrating the steps ofgrowing a semiconductor film on the porous structure of FIGS. 4(a)-4(d)and separating the semiconductor film from the semiconductor substrate;

FIGS. 6(a)-6(e) are cross-sectional views illustrating the sequence ofsteps for forming a multilayer porous structure in accordance with athird embodiment of the invention;

FIGS. 7(a)-7(b) are cross-sectional views illustrating the steps offorming a semiconductor film on the porous structure prepared in FIGS.6(a)-6(e) and separating the semiconductor film from the semiconductorsubstrate;

FIGS. 8(a)-8(f) are cross-sectional views illustrating anotherembodiment of the process for forming a multilayer porous structure on asemiconductor substrate, forming a semiconductor film on the porousstructure and separating the semiconductor film from the semiconductorsubstrate;

FIGS. 9(a)-9(d) are cross-sectional views illustrating anotherembodiment of the method of the present invention for forming amultilayer porous structure on a semiconductor substrate;

FIGS. 10(a)-10(d) are cross-sectional views illustrating formation of amultilayer semiconductor film on the porous structure shown in FIGS.9(a)-9(d) and separating of the multilayer film from the semiconductorsubstrate;

FIGS. 11(a)-11(c) are cross-sectional views illustrating formation of amultilayer porous structure on a semiconductor substrate;

FIGS. 12(a)-12(b) are cross-sectional views illustrating formation of amultilayer semiconductor film on the porous structure shown in FIGS.11(a)-11(c) and formation and patterning of an insulating layer thereonto define contact holes, respectively;

FIGS. 13(a)-13(b) are cross-sectional views illustrating formation ofelectrodes in the contact holes of the substrate prepared in FIG. 11(c)and attachment of a printed circuit board substrate to the electrodesand semiconductor substrate, respectively;

FIGS. 14(a)-14(b) are cross-sectional views illustrating separation ofthe created solar panel structure from the semiconductor substrate andattachment of a rear side metal electrode, respectively;

FIGS. 15(a)-15(d) are cross-sectional views illustrating formation of amultilayered porous structure in a semi-conductor substrate;

FIGS. 16(a)-16(b) are cross-sectional views illustrating formation of anepitaxially grown semiconductor thin film on the porous substrate ofFIG. 15(d) and separation of the semiconductor thin film from thesemiconductor substrate, respectively;

FIGS. 17(a)-17(e) are cross-sectional views illustrating formation ofanother porous structure on a semiconductor substrate, and formation ofan epitaxially grown semiconductor thin film thereon;

FIGS. 18(a)-18(e) are cross-sectional views illustrating formation ofanother porous structure on a semiconductor substrate and formation of athin film semiconductor thereon;

FIGS. 19(a) and 19(b) are cross-sectional views illustrating formationof an electrode structure on the semiconductor thin film and attachmentof conductor members and a transparent substrate to the solar panelpanel structure, respectively;

FIGS. 20(a)-20(b) are cross-sectional views illustrating separation ofthe solar panel structure from the semiconductor substrate andattachment of a back electrode, respectively;

FIGS. 21(a)-21(c) are side elevation views of a plurality of solar panelstructures showing the method of the present invention through anintermediate stage of formation;

FIGS. 22(a)-22(b) are side elevation views illustrating the methodshowing the completion steps for the solar panels being prepared inFIGS. 21(a)-21(c);

FIG. 23 is a schematic cross-sectional view of a micrograph of aprincipal part of a porous layer in the process of the present inventionshown in cross-section before heat treatment;

FIG. 24 is a schematic cross-sectional view of a micrograph of theprincipal part of the porous layer in the process of the presentinvention after heat treatment.

FIGS. 25(a)-25(e) are cross-sectional views illustrating formation of ahetero junction and porous structure in a semiconductor substrate inaccordance with an embodiment of the method for making a light emittingdiode of the present invention.

FIGS. 26(a)-26(d) are cross-sectional views illustrating the steps ofattaching electrodes and a support substrate and separating a diodesubstrate from the semiconductor substrate in accordance with anembodiment of the invention;

FIGS. 27(a)-27(c) are cross-sectional views illustrating the furthersteps of attaching a second array of electrodes and a second supportsubstrate in accordance with an embodiment of the invention; and

FIG. 28 is a process diagram of another embodiment of the process of thepresent invention (fourth). FIG. 28A and FIG. 28B are sectional views ofthe process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with an embodiment of the present invention, the surfaceof a semiconductor substrate may be changed by anodization to form aporous structure comprising two or more porous layers, each havingdifferent porosities. Then, a semiconductor film is epitaxially grown onthe surface of this porous layer. Thereafter, this epitaxialsemiconductor film is peeled or separated from the semiconductorsubstrate along a line of weakness formed in the porous structure toproduce the intended thin film semiconductor.

On the other hand, the remaining semiconductor substrate may berepeatedly used for the production of the above thin film semiconductor.Further, this semiconductor substrate per se, which becomes thin due tothe repeated use, can be used as the thin film semiconductor.

In the step of forming the porous layer, a low porosity layer is formedin the surface of the substrate. Thereafter, a high porosity layer isformed beneath an interface (in the present specification, the interfaceof the semiconductor substrate means an interface between thesemiconductor substrate, which is not made porous, and the porous layer)between the low porosity layer and the semiconductor substrate.

Further, in the step for forming the porous layer, for example, a lowporosity layer in the surface of the substrate, an intermediate porositylayer formed beneath the low porosity layer, having a higher porositythan that of the surface layer, and a high porosity layer formed in thisintermediate porosity layer or beneath the inter-mediate porosity layer,having a higher porosity than that of the intermediate porosity layercan be formed.

The porous layer can be formed by anodization. This anodization processcomprises at least two or more steps conducted with different currentdensities. Accordingly, in the anodization process, a step for anodizingthe surface of the semiconductor substrate with a lower current densityand a step for anodizing the same with a higher current densitythereafter are adopted.

For example, in the anodization, it is possible to adopt a step foranodizing the surface of the semiconductor substrate with a low currentdensity; a step for further anodizing the same with an intermediate lowcurrent density slightly higher than this low current density; and astep for further anodizing the same with a higher current density thanthis.

Further, in the anodization with a high current density, it is possibleto intermittently pass a current with a high current density.

Further, in the anodization with the intermediate low current density,the current density thereof can be gradually increased.

Further, the anodization can be performed in an electrolytic solutioncontaining hydrogen fluoride and ethanol or in an electrolytic solutioncontaining hydrogen fluoride and methanol.

Further, in the anodizing step, the composition of the electrolyticsolution can be changed when the current density is changed.

After forming the porous layer, the layer is preferably heated in ahydrogen gas atmosphere. Further, after forming the porous layer andbefore the heating step in the hydrogen gas atmosphere, preferably theporous layer is thermally oxidized.

Various shapes of the semiconductor substrate can be used for thepresent invention. For example, it can be shaped as a wafer, that is, adisk, or a column-like ingot having a curved surface obtained from apulling up of a single crystal.

The semiconductor substrate can be constituted by various semiconductorsubstrates such as a single crystal substrate of silicon Si, an Sipolycrystalline substrate in certain cases, or a compound semiconductorsubstrate, for example, a GaAs single crystal. A Si single crystalsubstrate is preferably used for the production of an Si single crystalthin film and a solar cell by an Si single crystal thin film.

Further, an n-type or a p-type impurity-doped semiconductor substrate oran intrinsic semiconductor substrate can be used. For anodization of thepresent invention, a semiconductor substrate having a low resistancedoped with a p-type impurity at a high concentration, i.e., a so-calledp+-type Si substrate, is desirably used. As a p+-type Si substrate, anSi substrate, in which boron, B, a p-type impurity, is doped to about10¹⁹ atoms/cm^(') and having a resistance thereof of about 0.01 to 0.02Ωcm, can be used. When the p+-type Si substrate is anodized, thin andlong fine pores, extending in a vertical direction generallyperpendicular to the surface of the substrate, are formed. In theanodization process, the substrate is changed to form a porous layerwhile maintaining its crystallinity, therefore a desirable porous layeris formed.

As mentioned above, the semiconductor film is epitaxially grown on theporous layer while maintaining its crystallinity. Due to this, it ispossible to constitute the single crystal semiconductor film. Further,when constructing a solar cell or other devices, the grown semiconductorfilm may comprise a multi-layer semiconductor film.

According to the present invention, the semiconductor film epitaxiallygrown on the porous layer is separated from the semiconductor substratethrough a directed fracture of the porous layer. Prior to peeling orseparation, a support substrate comprised, for example, a flexibleplastic sheet can be bonded onto the semiconductor film as a supportsubstrate. The semiconductor film can be peeled from the semiconductorsubstrate together with the support substrate via the porous layerformed on this semiconductor substrate.

This support substrate is not limited to a flexible sheet. For thesupport substrate, it is possible to use a glass substrate, a resinsubstrate, or a flexible or rigid transparent printed circuit board towhich desired printed interconnections are applied.

In the surface of the semiconductor substrate, two or more porous layershaving different porosities are formed. The first porous layer of theoutermost surface is preferably formed as a dense porous layer which hasa relatively small porosity to provide improved growth of an epitaxialsemiconductor film on this porous layer. By forming a second porouslayer having a relatively high porosity spaced inwardly, adjacent to thesurface of the substrate and first porous layer, the mechanical strengthfalls due to the high porosity of the second layer itself, or expresseddifferently, the bond between the first and second porous layers becomesfragile due to the strain caused by the difference of the latticeconstants of each of these layers. Due to this, the peeling of theepitaxial semiconductor film, that is, separation, can be easily carriedout via the high porosity layer. For example, it becomes also possibleto form a porous layer which is so weak that it can be separated byultrasonic irradiation and excitation of the substrate.

The high porosity layer formed is easier to peel because of the largerthe porosity thereof, but if this porosity is too large, breakage in thehigh porosity layer may occur before the peeling step of thesemiconductor film on the porous layer. Therefore, the porosity in thishigh porosity layer is preferably made 40 percent to 70 percent.

Further, with the increasing of the porosity, the strain becomes large.If the influence of this strain becomes larger in the surface layer ofthe first porous layer, cracks may occur in the surface layer. Further,if the influence of the strain reaches up to the surface of the porouslayer, crystal defects may be caused in the semiconductor filmepitaxially grown on the porous layer. Therefore, in the porous layer,an intermediate porosity layer, having an intermediate porosity which ishigher than that of the surface layer, but lower than that of the highporosity layer, is preferably formed between the high porosity layer andthe surface layer. In this case, the intermediate porosity layer can bea buffer layer for relieving the strain. Accordingly, the porosity ofthe high porosity layer can be made large enough so that the peeling ofthe epitaxial semiconductor film can be reliably carried out.Additionally, an epitaxial semiconductor film which is excellent incrystallinity, can be formed on the porous layer.

The anodization, for changing the surface of the semiconductor substrateto form a porous layer, can be performed by a well known method. Forexample, a double cell method shown in Ito et al., "Surface Technology",vol. 46, no. 5, pp. 8 to 13, 1995 (Anodization of Porous Si) isapplicable. A schematic structural view shown in FIG. 1 is used for thedouble cell method. In this method, an electrolytic solution cell 1having a first and second cells 1A and 1B is used. The semiconductorsubstrate 11, on which the porous layer is to be formed, is arrangedbetween the two cells 1A and 1B. Two platinum electrodes 3A and 3Bconnected to a DC current source 2 are arranged between the first andsecond cells 1A and 1B. In the first and second cells 1A and 1B, anelectrolytic solution 4 contains, for example, hydrogen fluoride HF andethanol C₂ H₅ OH or hydrogen fluoride HF and methanol CH₃ OH. Thesemiconductor substrate 11 is arranged so that its two surfaces are incontact with the electrolytic solution 4 in the first and second cells1A and 1B. The two platinum electrodes 3A and 3B are arranged immersedin the electrolytic solution 4. Then, a current is supplied between thetwo electrodes 3A and 3B by the DC current source 2 using the electrode3A as a cathode. The current is supplied so as the surface of thesemiconductor substrate 11 facing the electrode 3A is corroded andbecomes porous.

According to this double cell method, it is unnecessary to coat an ohmicelectrode on the semiconductor substrate and introduction of theimpurities into the semiconductor substrate from this ohmic electrode isavoided.

The structure of the porous layer can be changed by selecting theconditions in the anodization process, whereby the crystallinity of thesemiconductor film, to be formed, on the porous layer and the peelingproperty change.

In the process of the present invention, as mentioned above, a porouslayer comprising two or more layers having different porosities isformed. In this case, a multi-step anodizing method comprising two ormore steps conducted by different current densities is used. Morespecifically, to prepare a relatively dense, low porosity layer havingsmall fine pores in the surface of the semiconductor substrate, thefirst anodization step is applied with a lower current density. The filmthickness of the porous layer is proportional to the current supplytime, therefore, in this step, the anodization time is selected to forma desired film thickness. Thereafter, second anodization is carried outwith a higher current density so as to form a high porosity layerbeneath the low porosity layer formed. As a result, a porous layercomprising at least a low porosity layer having a lower porosity and ahigh porosity layer having a higher porosity is formed.

In this case, a large strain may occur near the interface between thelow porosity layer and the high porosity layer due to the difference ofthe lattice constants. When this strain reaches a certain value or more,the porous layers separate into two. Accordingly, by form-ing the porouslayers under anodizing conditions below and near the critical conditionwhich causes the separation due to the strain or the lowering of themechanical strength, the semiconductor film epitaxially grown on aporous layer can be easily separated via the porous layer.

The first anodization step can be carried out with the lower currentdensity of about 0.5 to 10 mA/cm² for a period of about 1 to about 60minutes, preferably 2 to 20 minutes, by using a p-type silicon singlecrystal substrate of 0.01 to 0.02 Ωcm and a ratio of HF (49% solution)and C2H5OH (95% solution) of 1:1 (volume ratio) (hereinafter, HF andC2H5OH indicates the volume ratio in the 49% solution and 95% solution,respectively). Further, the second anodization can be carried out with ahigher current density of about 40 to 300 mA/cm² for a period of about 1to 10 seconds, preferably a time of approximately 3 seconds.

In the first and second anodization steps, the strain caused in the highporosity layer may become considerably large, therefore the influence ofthis strain may reach up to the low porosity layer. In this case, asmentioned above, the crystal defects may be introduced in the epitaxialsemiconductor film formed on the porous layer. Therefore, in the porouslayer, an intermediate porosity layer, having a higher porosity than thesurface layer, but a lower porosity than that of the high porositylayer, is preferably formed between the low porosity layer and the highporosity layer, as a buffer layer for relieving the strain generated bythem. More specifically, a first anodization with a lower currentdensity is carried out at first, a second anodization of a slightlyhigher cur-rent density than that in the first anodization is carriedout, and then a third anodization is carried out with a considerablyhigher current density than them. The conditions of the firstanodization are not particularly limited, but when a p-type siliconsingle crystal substrate of 0.01 to 0.02 Ωcm is used and an electrolyticsolution of HF:C₂ H₅ OH=1:1 is used as the electrolytic solution,preferably the first, second and third anodization can be carried outwith a current density of about 0.5 to 3 mA/cm2, 3 to 20 mA/cm2, and 40to 300 mA/cm2, respectively. For example, when the anodization iscarried out with a current density of 1 mA/cm2, 7 mA/cm2, and 200mA/cm2, the porosity becomes about 16%; 26% and 60 to 70%, respectively.According to the above mentioned method, the semiconductor layer havinggood crystallinity can be epitaxially grown on the porous layer.

Further, in the anodization process, the intermediate porosity layer canbe formed beneath the low porosity layer while the porosity of the lowporosity layer is held as it is. Accordingly, the porous layer becomes atwo-layer structure of the low porosity layer and the intermediateporosity layer. Further, in the third anodization step, if the currentdensity is selected about 90 mA/cm2 or more, a high porosity layer isformed within the intermediate porosity layer, though the principle isnot clear.

Further, in the step forming the intermediate porosity layer, bychanging the current density gradually or stepwise, an intermediateporosity layer, with a porosity which rises gradually or stepwise fromthe low porosity layer toward the high porosity layer, is formed betweenthe low porosity surface layer and the high porosity layer. As a result,the strain between the low porosity layer and the high porosity layer isrelieved more and an epitaxial semiconductor film having a goodcrystallinity can be further reliably formed on the porous layer.

The separation will occur by a large strain caused by the differentlattice constants at the interface between the peeling layer (separationlayer) of the high porosity layer and a buffer layer of the intermediateporosity layer. If some special action is taken in third anodizationstep, the separation becomes even easier. This is obtained by applyingthe current intermittently in the third anodization step with the highercurrent density, for example, not supplying the current continuously for3 seconds, but by the alternative steps comprising supplying current for1 second and then stopping the current for a predetermined time, forexample, about 1 minute. By intermittently supplying current in discretesteps, the high porosity layer, as the separation layer, can be formedbeneath the intermediate porosity layer. In this case, the porous layer,remaining with the semiconductor layer after separation, can be removedby the electrolytic grinding or other chemical or mechanical etchingmethods.

As mentioned above, by forming the high porosity layer beneath theintermediate layer, a distance between the high porosity layer, in whichthe strain occurs, and the surface of the porous layer is larger and thebuffering effect by the intermediate porosity layer becomes larger, thusa semiconductor film having a good crystallinity can be formed. Further,when the high porosity layer is formed beneath the intermediate porositylayer, the thickness of the entire porous layer can be reduced, and thethickness of the semiconductor substrate consumed for forming thisporous layer can be reduced, and the number of times of repeated use ofthis semiconductor substrate can be made larger.

Accordingly, by the selection of the anodization conditions, it ispossible to introduce a large strain in the separation layer and inaddition, to keep the influence of this strain from reaching theepitaxially grown surface of the semiconductor film.

In order to perform the epitaxial growth of the semiconductor on theporous layer with a good crystallinity, it is desired to form thesurface of the porous layer with the small fine pores, which serve asthe seeds of the crystal growth on the porous layer. To make the smallfine pores, a high HF concentration electrolytic solution can be used.In this case, in the low current anodization step for forming the lowporosity layer, an electrolytic solution having a high HF concentrationis used. Next, the intermediate porosity layer which serves as thebuffer layer is formed, then the HF concentration of the electrolyticsolution is lowered and the anodization with a high current density isfinally carried out. By this process, it is possible to make the size ofthe fine pores of the surface layer very small, whereby an epitaxialsemiconductor film having a good crystallinity can be formed on theporous layer. In addition, in the high porosity layer, the porosity canbe raised to a necessary sufficiently high level, so the peeling of theepitaxial semiconductor film can be carried out well.

Regarding the change of the electrolytic solution in the anodizationprocess, in the first step forming the surface low porosity layer, theanodization is carried out using an electrolytic solution of, forexample, HF:C₂ H₅ OH=2:1; in the second step forming the intermediateporosity layer serving as the buffer layer, the anodization is carriedout using an electrolytic solution of a slightly low HF concentration,for example, HF:C₂ H₅ OH=1:1; and further, in the third step forming thehigh porosity layer, the anodization with a high current density iscarried out using an electrolytic solution with a reduced HFconcentration of, for example, HF:C₂ H₅ OH=1:1 to 1:2.

In the anodization process, when changing the current density in theperiod from the first step to second step, the current supply can bestopped before the second step. Or after the first step, the second stepcan be conducted without interruption of the current supply.

Further, the anodization process can be conducted in a dark place wherethe light is blocked, to make the unevenness of the surface of theporous layer smaller and raise the crystallinity of the semiconductorfilm epitaxially grown on the porous layer.

Additionally, the anodized porous layer of silicon can be utilized as alight emitting diode. In this case, the anodization is preferablycarried out while irradiating light to rise the light emittingefficiency. Further, oxidizing the anodized porous layer, a blue shiftof the wavelength of emitted light occurs. Further, the semiconductorsubstrate, which may be p-type or n-type, preferably has a highresistance so as not to introduce the impurities.

Employing the above mentioned process, a semiconductor substrate formedon the surface (one surface or both surfaces) of the porous layer can beobtained. Further, the thickness of the entire porous layer is notparticularly limited, but the thickness can be made 1 to 50 μm,preferably 3 to 15 μm, usually about 8 μm. The thickness of the entireporous layer is preferably reduced as much as possible so that thesemiconductor substrate may be repeatedly used as much as possible.

Additionally, the porous layer is preferably annealed preceding theepitaxial growth of the semiconductor on the porous layer. Thisannealing can be a hydrogen annealing which can be conducted by a heattreatment in a hydrogen gas atmosphere. By the hydrogen annealing, thenatural oxidized film formed on the surface of the porous layer can becompletely removed and the oxygen atoms in the porous layer can beremoved as much as possible. As a result, the surface of the porouslayer becomes smooth, and an epitaxial semiconductor film having a goodcrystallinity can be formed. Simultaneously, by this annealing, thestrength of the interface between the high porosity layer and theintermediate porosity layer can be further weakened, and the separationof the epitaxial semiconductor film from the substrate can be moreeasily carried out. This hydrogen annealing can be carried out in atemperature range of from about 950° C. to 1150° C.

Further, oxidizing the porous layer at a low temperature before thehydrogen annealing, the internal portion of the porous layer isoxidized. Due to this no large structural change will occur in theporous layer even if the heat treatment in the hydrogen gas atmosphereis applied. Accordingly, the strain between the high porosity andintermediate porosity layers is isolated in an area remote or separatefrom the surface of the first porous layer, and an epitaxialsemiconductor film having a good crystallinity can be formed. In thiscase, the low temperature oxidation can be carried out in a dryoxidation atmosphere at 400° C. for about 1 hour.

After the hydrogen annealing, as mentioned above, a semiconductor can beepitaxially grown on the surface of the porous layer. In the epitaxialgrowth of this semiconductor, the porous layer formed in the surface ofthe single crystal semiconductor substrate maintains its crystallinityalthough it is porous. Therefore, the epitaxial growth on this porouslayer is possible. The epitaxial growth on the surface of this porouslayer can be carried out by a CVD process at a temperature of, forexample, 700° C. to 1100° C.

Further, in both of the hydrogen annealing and the epitaxial growth ofthe semiconductor, as the method of heating the semiconductor substrateto the predetermined temperature, the so-called susceptance heatingsystem or the conduction heating system directly supplying a currentthrough the semiconductor substrate per se for heating can be adopted.

The above-mentioned semiconductor film epitaxially grown on the porouslayer can be a single layer semiconductor film or multi-layersemiconductor film by lamination of a plurality of semiconductor layers.Further, this semiconductor film can be the same substance as that forthe semiconductor substrate or different substances. As thesemiconductor film, various types of the semiconductor film or films canbe used. For example, a single crystal Si semiconductor film, a compoundsemiconductor of GaAs etc., a Si compound, for example Si_(1-y) Ge_(y),or films suitably combined and stacked the same can be used.

Further, in a case the semiconductor film is a compound semiconductor, acompound semiconductor substrate can be used as the semiconductorsubstrate. In this case, by anodizing the compound semiconductorsubstrate, the compound semiconductor film can be formed on the porouslayer. When a compound semiconductor is epitaxially grown on the porouslayer made of the compound semiconductor, the lattice mismatch can bereduced in comparison with the case where the compound semiconductor isepitaxially grown on a Si semiconductor substrate, and therefore a thinfilm compound semiconductor having a good crystallinity can be formed.

Further, n-type or p-type impurities can be introduced into thesemiconductor film formed on the porous layer at the time of theepitaxial growth thereof. Alternatively, it is also possible tointroduce the impurities in the entire surface or selectively by an ionimplantation, diffusion, etc. after forming the epitaxial semiconductorfilm. In this case, the conductivity type and the concentration and typeof impurities are selected in accordance with the object of use thereof.

Further, the thickness of the epitaxial semiconductor film can beappropriately selected in accordance with the purpose of the thin filmsemiconductor. For example, when forming a semiconductor integratedcircuit on the thin film semiconductor, since the active layer of thesemiconductor element has a thickness of about several micrometers, thesemiconductor film can be formed to a thickness of for example about 5μm.

When epitaxially growing a semiconductor film made by single crystalsilicon to form the thin film semiconductor solar cell, as thesemiconductor film, for example, a p⁺ -type semiconductor layer, a p⁻-type semiconductor layer, and an n⁺ -type semiconductor layer, in thisorder on the porous layer can be used. The impurity concentration andthe film thickness of these layers are not particularly limited, but forexample, preferably, the p⁺ -type semiconductor layer is given a filmthickness within a range of from 0 to 1 μm, typically about 0.5 μm, anda boron B. as a p-type impurity, concentration within a range of from10¹⁸ to 10²⁰ atoms/cm³, typically about 10¹⁹ atoms/cm³ ; the p-typesemiconductor layer is given a film thickness within a range of from 1to 30 μm, typically about 5 μm, and a boron concentration within a rangeof from 10¹⁴ to 10¹⁷ atoms/cm³, typically about 10¹⁶ atoms/cm³ ; and then⁺ -type semiconductor layer is given a film thickness within a range offrom 0.1 to 1 μm, typically about 0.5 μm, and a concentration ofphosphorus, P. or arsenic, As, within a range of from 10¹⁸ to 10²⁰atoms/cm³, typically about 10¹⁹ atoms/cm³.

Further, the semiconductor film can be constituted by epitaxial growthof a p⁺ -type Si layer, p-type Si_(1-x) Ge_(x) graded layer, undopedSi_(1-y) Ge_(y) layer, n-type Si_(1-x) Ge_(x) graded layer, and n⁺ -typesilicon layer in this order and this used to prepare a double heterostructure solar cell. As typical examples of the layers for constitutingthis double hetero structure, preferably, for the p⁺ -type Si layer, theimpurity concentration is about 10¹⁹ atoms/cm³ and the film thickness isabout 0.5 μm; for the p-type Si_(1-x) Ge_(x) graded layer, the impurityconcentration is about 10¹⁶ atoms/cm³ and the film thickness is about 1μm; for the undoped Si_(1-y) Ge_(y) layer, y is 0.7 and the filmthickness is about 1 μm; for the n-type Si_(1-x) Ge_(x) graded layer,the impurity concentration is about 10¹⁶ atoms/cm³ and the filmthickness is about 1 μm; and for the n⁺ -type Si layer, the impurityconcentration is about 10¹⁰ cm⁻³ and the film thickness is about 0.5 μm.Further, the ratio of composition x of Ge in the p-type and n-typeSi_(1-x) Ge_(x) graded layers is preferably gradually increased from x=0of the layers existing on both sides to y of the undoped Si_(1-y)Ge_(y). Due to this, the lattice constants match at the interfaces,whereby a good crystallinity can be obtained.

In such a double hetero structure solar cell of, carriers and light canbe effectively confined in the undoped Si_(1-y) Ge_(y) layer, andtherefore a high conversion efficiency can be obtained.

Alternatively, the processing for a solar cell can be carried out beforethe peeling from the semiconductor substrate. In this case, the peelingprocess will be carried out after the attachment of a support substrateon the semiconductor film formed on the porous layer and thesemiconductor film is peeled from the semiconductor substrate togetherwith the support substrate.

The support substrate in the solar cell can be constituted by varioussubstrates for example a glass sheet such as window glass, a metalsubstrate, a ceramic substrate, or a flexible substrate comprised of atransparent resin film or sheet (hereinafter, simply referred to as asheet) etc.

Next, the steps for constructing the solar cell will be explained. It ispossible to perform these steps after or before peeling thesemiconductor film from the semiconductor substrate.

In the process for constructing the solar cell, as mentioned above,multi-layer silicon semiconductor film is epitaxially grown on thesemiconductor substrate on the surface of which the porous layer isformed. Next, for example, thermal oxidation processing is carried outto form an oxide film having thickness of about 10 to 200 nm on thesurface of the semiconductor film. Then, the oxide film of the surfaceof the semiconductor film is patterned to form an interconnection layerby the photolithography. Alternatively, it is also possible to formopenings at only the portions where connection to the semiconductor filmis necessary. Thereafter, for example a conductive layer forconstituting the electrode and interconnection layer, for example, asingle metal layer such as Al or multiple metal layers formed bylamination of a plurality of metal layers are finally formed on theentire surface by vapor deposition etc. and this is patterned to formthe required electrodes and interconnection pattern by thephotolithography and the etching. Alternatively, the electrodes andinterconnection pattern can be formed by a printing method.

Further, a so-called printed circuit board made of a transparent resinsheet, on which the required electrodes and interconnection pattern,i.e., so-called printed interconnections are formed, is prepared inadvance and this printed circuit board is attached on the semiconductorfilm formed on the porous layer to form the electrical contact at thecorresponding parts. At this time, the electrodes of the semiconductorfilm and the printed circuit are joined by for example solder. Further,parts other than the electrodes can be bonded by using a transparentbinder such as an epoxy resin.

In this way, the adhesion of a printed circuit board and thin filmsingle crystal silicon (Si), which had not been possible in the past,can be carried out extremely easily in the present invention. Further,in the present invention, the support substrate is not limited to aprinted circuit board--a transparent resin sheet can be adhered as well.After the support substrate such as a printed circuit or a transparentresin sheet is adhered, tensile stress may be applied with thesemiconductor substrate, thereby causing destruction in the highporosity layer or at an interface between the high porosity layer andintermediate porous layer or interface between the high porosity layerand the semiconductor substrate, the epitaxial semiconductor film to beeasily peeled from the semiconductor substrate the support substrate. Inthis way, a flexible solar cell, comprised of a thin film semiconductoron the support substrate such as a printed circuit board, can beobtained.

After the peeling process, the porous layer sometimes remains on theback surface of the semiconductor film opposite to the supportsubstrate. In this case, it is possible to remove this porous layer byfor example etching. Alternatively, a metal film such as a silver pasteused as the other ohmic electrode or the light reflecting film can beformed on this porous layer without removing. This light reflectingsurface will improve the opto-electric conversion efficiency. Further,it is also possible to adhere a metal sheet or form a resin layer on theback surface of the semiconductor film as a protective layer.

On the other hand, the semiconductor substrate from which thesemiconductor film is peeled may be ground at its surface subjected to asimilar operation repeatedly to form a porous layer and solar cells etc.The thickness of the semiconductor substrate can be made for exampleabout 200 to 300 μm, while the thickness of the semiconductor substrateconsumed per preparation of the solar cell is about 3 to 20 μm, so eventhe thickness consumed after 10 repeated uses is about 30 to 200 μm.Therefore, the semiconductor substrate can be sufficiently repeatedlyutilized. Accordingly, according to the process of the presentinvention, an expensive single crystal semiconductor substrate can berepeatedly used, and therefore a solar cell can be produced with a lowenergy while reducing the costs. Further, the semiconductor substratemade sufficiently thin by this repeated procedure can itself be used toconstitute a solar cell.

Next, the embodiments of the present invention will be explained.However, the present invention is not limited to these embodiments.First, an explanation will be made of an embodiment of the process ofproduction of a thin film semiconductor. according to the presentinvention.

[Embodiment 1]

FIG. 2 and FIG. 3 show process diagrams of this Embodiment 1. First, awafer-like semiconductor substrate 11 made of a single crystal Si, dopedwith boron B at a high concentration, having a resistivity of forexample 0.01 to 0.02 Ωcm was prepared (FIG. 2A).

Then, the surface of this semiconductor substrate 11 was anodized toform a porous layer in the surface of the semiconductor substrate 11. Inthis embodiment, the anodization was carried out by employing ananodizing device having a double cell structure explained in FIG. 1.Namely, the semiconductor substrate 11 made of the single crystal Si wasarranged between the first and second cells 1A and 1B, and anelectrolytic solution made of HF:C₂ H₅ OH=1:1 was filled into the twocells 1A and 1B. Then, a current was passed between the Pt electrodes 3Aand 3B immersed in the electrolytic solutions of the electrolyticsolution cells 1A and 1B by a DC current source 2.

First, current was supplied with a low current density of 7 mA/cm² for13 minutes. By this step, a surface layer 12S having a porosity of 26%and a thickness of about 10 μm was formed (FIG. 2B).

The current supply was stopped once, then a current was supplied with ahigh current density of 200 mA/cm² for 3 seconds. By this step, a highporosity layer 12H having a porosity of about 60% higher than that ofthe surface layer 12S was formed in the surface layer 12S. Accordingly,the high porosity layer was sandwiched between the previously formedsurface layer 12S (FIG. 2C). In this way, a porous layer 12 made of thesurface layer 12S and the high porosity layer 12H was formed.

In the porous layer 12, the surface layer 12S and the high porositylayer 12H have greatly different porosities, therefore a large strain isintroduced at the interface between the surface layer 12S and the highporosity layer 12H and the strength becomes extremely weak around thevicinity of the interface. Accordingly, a line of weakness is defined ator about the interface which is useful to facilitate targeted separationof grown semiconductor films from the substrate in later steps.

After forming the porous layer 12, in the Si epitaxial growing device, aheat treatment, that is, an annealing at 1100° C., was carried out forthe semiconductor substrate 11 in an H₂ atmosphere under a normalpressure. The heating step was performed by raising the heatingtemperature from room temperature to 1100° C. in about 20 minutes andthen holding at 1100° C. for about 30 minutes. By this annealing in H₂,the surface of the porous layer 12 became smooth, and the strength nearthe interface between the intermediate porosity layer 12M and the highporosity layer 12H was made further fragile.

Thereafter, the temperature was reduced from the annealing temperatureof 1100° C. to 1030° C. in the H₂ atmosphere and epitaxial growth of Siwas carried out for 17 minutes by using SiH₄ gas as a source gas. As aresult, an epitaxial semiconductor film 13 made of single crystal Sihaving a thickness of about 5 μm was formed on the surface of the porouslayer 12 (FIG. 3A).

The epitaxial semiconductor film 13 was peeled from the semiconductorsubstrate 11 in the next step. For this peeling step, a binder 14 wascoated on the surface of the epitaxial semiconductor film 13 and theback surface of the semiconductor substrate 11, respectively, and aflexible support substrate 15 made of a PET (polyethylene terephthalate)sheet was adhered by these binders 14 (FIG. 3B). The bonding strength ofthe support substrate 15 by this binder 14 was selected to be a higherstrength than the separation strength in the porous layer 12.

An external outward and opposed pulling force for separating the supportsubstrates 15 from each other was applied to the two substrates 15. Bythis, separation occurred along the line of weakness in the fragileporous layer 12 with peeling occurring in the high porosity layer 12H,or, at or in the vicinity of an interface with the high porosity layer12H and the epitaxial semiconductor film 13 was peeled from thesemiconductor substrate 11 (FIG. 3C).

The thin film semiconductor 23 is constituted by the epitaxialsemiconductor film 13 separated in this way (FIG. 3D). In this example,any remaining porous layer with the thin film semiconductor 23 wasremoved by chemical or mechanical etching.

[Embodiment 2]

FIGS. 4-5 are process diagrams of the Embodiment 2. First, similar toEmbodiment 1, a wafer-like semiconductor substrate 11 made of singlecrystal Si doped with boron B at a high concentration and having aresistivity of for example 0.01 to 0.02 Ωcm was prepared (FIG. 4A).

Next, the surface of this semiconductor substrate 11 was anodized toform a porous layer in the surface of the semiconductor substrate 11.Also in this Embodiment 2, similar to Embodiment 1, the anodizing deviceof the double cell structure explained referring to FIG. 1 was used. Andan electrolytic solution made of HF:C₂ H₅ OH=1:1 was filled into both ofthe first and second cells 1A and 1B. Then, a current was passed betweenthe Pt electrodes 3A and 3B immersed in the electrolytic solutions ofthe electrolytic solution cells 1A and 1B by a DC current source 2.

In this Embodiment 2, first, current was supplied for 8 minutes with alow current density of 1 mA/cm². By this, a dense surface layer 12Shaving a porosity of 16% and a thickness of 1.7 μm having a very smallpore diameter in comparison with the surface layer 12S in Embodiment 1was formed (FIG. 4B). The current supply was stopped once, then acurrent was supplied with a current density of 7 mA/cm² for 8 minutes.By this, an intermediate porosity layer 12M of a porosity of 26% andthickness of 6.3 μm having a higher porosity in comparison with thesurface layer 12S was formed beneath of the surface layer 12S (FIG. 4C).The current supply was stopped once again, then a current was suppliedwith a high current density of 200 mA/cm² for 3 seconds. By this, in theintermediate porosity layer 12M, a high porosity layer 12H, given ahigher porosity than this intermediate porosity layer 12M, that is,having a porosity of about 60% and thickness of 0.05 μm, was formed(FIG. 4D). In this way, a porous layer 12 comprising the surface layer12S, the intermediate porosity layer 12M, and the high porosity layer12H was formed.

In the porous layer 12, the porosity greatly differs between theintermediate porosity layer 12M and the high porosity layer 12H.Therefore, a large strain and an associated line of weakness isintroduced at and the vicinity of the interface of these intermediateporosity layer 12M and high porosity layer 12H, thus the strength aroundthis interface becomes extremely weak.

After the formation of the porous layer 12, the annealing, the epitaxialgrowth of Si, and the peeling are carried out in the same way as indescribed in Embodiment 1.

More particularly, in the ordinary pressure Si epitaxial growing device,first, the semiconductor substrate 11 was annealed in an H₂ atmosphere.The annealing, that is, the heat treatment, was performed by raising theheating temperature from room temperature to 1100° C. in about 20minutes, then holding at 1100° C. for about 30 minutes. In this H₂annealing, the surface layer 12S becomes smoother if the fine pores ofthe porous layer are small. Therefore, the surface layer 12S havingsmall fine pores becomes smoother by this annealing in H₂ and thestrength near the interface between the intermediate porosity layer 12Mand the high porosity layer 12H was made further fragile.

Thereafter, the temperature was reduced from the annealing temperatureof 1100° C. to 1030° C. in H₂, and the epitaxial growth of Si wascarried out for 17 minutes by using SiH₄ gas as the source gas. By doingthis, an epitaxial semiconductor film 13 made of single crystal Sihaving a thickness of about 5 μm was formed on the surface layer 12S(FIG. 5A).

After the above described step, a binder 14 was coated on the surface ofthe semiconductor film 13 and the back surface of the semiconductorsubstrate 11, the PET sheets (not illustrated) were bonded by a strongerbonding strength than the separation strength in the porous layer 12 bythe binder 14. Then, an external force for separating the semiconductorfilm 13 from the semiconductor substrate 11 was applied similar toEmbodiment 1. By this, in the fragile porous layer 12, peeling occurs inthe high porosity layer 12H or at the interface between the intermediateporosity layer 12M and the high porosity layer 12H or the vicinitythereof, and the epitaxial semiconductor film 13 is separated from thesemiconductor substrate 11 (FIG. 5B).

The thin film semiconductor 23 is constituted by the epitaxialsemiconductor film 13 separated in this way. In this example, the porouslayer peeled with the thin film semiconductor 23 was again removed bychemical or mechanical etching.

In this Embodiment 2, a surface layer 12S which has a small and denseporosity, and becomes smoother by the annealing in H₂. Therefore, thesemiconductor film 13 epitaxially grown on the surface layer 12S isformed to have a more excellent crystallinity.

Nevertheless, the porosity difference between the surface layer 12S andthe high porosity layer 12H is large, the intermediate porosity layer12M having an intermediate porosity is provided as a buffer layerbetween the high porosity layer 12H and the surface layer 12S,therefore, the influence of the strain due to the high porosity layer12H can be effectively reduced.

[Embodiment 3]

FIG. 6 and FIG. 7 are process diagrams of this Embodiment 3. First,similar to Embodiments 1 and 2, a wafer-like semiconductor substrate 11made by single crystal Si doped with boron B at a high concentration andhaving a resistivity of for example 0.01 to 0.02 Ωcm was prepared (FIG.6A).

Next, the surface of this semiconductor substrate 11 was anodized toform a porous layer in the surface of the semiconductor substrate 11. Inthis Embodiment 3, similar to Embodiments 1 and 2, an anodizing deviceof a double cell structure explained referring to FIG. 1 was used. Andan electrolytic solution made of HF:C₂ H₅ OH=1:1 was filled into both ofthe first and second cells 1A and 1B. Then, a current was passed betweenthe Pt electrodes 3A and 3B immersed in the electrolytic solutions ofthe electrolytic solution cells 1A and 1B by a DC current source 2.

In this Embodiment 3, after the above described step, the current wassupplied for 8 minutes with a low current density of 1 mA/cm². By this,similar to Embodiment 2, a dense surface layer 12S having a very smallpore diameter was formed (FIG. 6B).

The current supply was stopped once, then, in this Embodiment 3, acurrent was supplied with a current density of 4 mA/cm² for 3 minutes.By this, a first intermediate porosity layer 12M1 of a porosity of 22%and thickness of 1.8 μm having a higher porosity in comparison with thesurface layer 12S was formed beneath the surface layer 12S (FIG. 6C).

The current supply was stopped once again, then a current was furthersupplied with a current density of 10 mA/cm² for 6 minutes. By this, asecond intermediate porosity layer 12M2 having a porosity of about 30%and a thickness of 6.6 μm was formed beneath the first intermediateporosity layer 12M1 (FIG. 6D).

Further, the current supply was stopped once, then a current wassupplied with a high current density of 200 mA/cm² for 3 seconds. Bythis, in the second intermediate porosity layer 12M2, a high porositylayer 12H given a higher porosity than this intermediate porosity layer12M2, that is, having a porosity of about 60% and thickness of about 0.5μm, was formed (FIG. 6E). In this way, a porous layer 12 comprising thesurface layer 12S, the first and second intermediate porosity layers12M1 and 12M2, and the high porosity layer 12H was formed.

In the porous layer 12, the porosity greatly differs between the secondintermediate porosity layer 12M2 and the high porosity layer 12H,therefore a large strain is introduced at or the vicinity of theinterface of the second intermediate porosity layer 12M2 and the highporosity layer 12H, and thus the strength at this location becomesextremely weak.

After forming the porous layer 12, annealing is carried out similar toEmbodiments 1 and 2, the epitaxial semiconductor film 13 is formed byepitaxial growth of Si (FIG. 7A), and the bonding of a PET sheet (notillustrated) serving as the support substrate is carried out. Thepeeling of the epitaxial semiconductor film 13 and the semiconductorsubstrate 11 is carried out by the breakage of the high porosity 12H ofthe porous layer 12 or the vicinity thereof (FIG. 7B).

The thin film semiconductor 23 is formed by the epitaxial semiconductorfilm 13 in this way.

In this Embodiment 3, between the high porosity layer 12H and thesurface layer 12S, the first and second inter-mediate porosity layers12M1 and 12M2 having porosities which rise toward the high porositylayer 12H are provided. These intermediate porosity layers act as bufferlayers. Therefore, the influence of the strain due to the high porositylayer 12 can be more effectively reduced.

[Embodiment 4]

In this embodiment, similar to Embodiment 2 explained referring to FIG.4 and FIG. 5, the surface of the single crystal Si semiconductorsubstrate 11 is anodized to form the porous layer 12 comprised of thesurface layer 12S, the intermediate porosity layer 12M, and the highporosity layer 12H formed in this intermediate porosity layer 12M andthe epitaxial semiconductor film constituting the intended thin filmsemiconductor is epitaxially grown on this, but in this embodiment, thesurface layer 12S and the intermediate porosity layer 12M were formed bya continuous anodization method in which the amount of current suppliedis varied.

Also in this embodiment, similar to Embodiments 1 and 2, a semiconductorsubstrate 11 made of single crystal Si doped with boron B and having aresistivity of for example 0.01 to 0.02 Ωcm was prepared (FIG. 4A).

Then, similar to Embodiments 1 and 2, an anodizing device of a doublecell structure explained referring to FIG. 1 was used. An electrolyticsolution made of HF:C₂ H₅ OH=1:1 was filled into both of the first andsecond cells 1A and 1B. Then, current was passed between the Ptelectrodes 3A and 3B immersed in the electrolytic solution of theelectrolytic solution cells 1A and 1B by a DC current source 2.

Also in this Embodiment 4, first, current was supplied for 8 minuteswith a low current density of 1 mA/cm². By this, a surface layer 12Shaving a porosity of 16% and a thickness of 1.7 μm was formed (FIG. 4B).

Next, in this embodiment, after forming this surface layer 12S, withoutstopping the current supply, the anodization was carried out graduallychanging the amount of current supply from above 1 mA/cm² to 10 mA/cm²in about 16 minutes, by increasing the current at a rate of for example1 mA/cm² per minute. By this, an intermediate porosity layer 12M havinga thickness of about 6.8 μm and changed in porosity from about 16% to30% was formed (FIG. 4C).

Thereafter, the current supply was stopped once, then current wassupplied with a high current density of 200 mA/cm² for 3 seconds. Bythis, in the intermediate porosity layer 12M, a high porosity layer 12Hgiven a higher porosity than this intermediate porosity layer 12M,having a porosity of about 60% and thickness of 0.5 μm, was formed (FIG.4D). In this way, a porous layer 12 comprising the surface layer 12S,the intermediate porosity layer 12M, and the high porosity layer 12H wasformed.

In the porous layer 12 formed in this way, the porosity greatly differsbetween the intermediate porosity layer 12M and the high porosity layer12H, therefore a large strain is applied at the interface of theseintermediate porosity layer 12M and high porosity layer 12H and thevicinity of the interface, thus the strength around here becomesextremely weak.

After forming the porous layer 12, annealing similar to Embodiments 1and 2 is carried out in a Si epitaxial growing device in H₂ atmosphereunder the normal pressure to make the surface layer 12S of the porouslayer 12 smooth and weaken the strength near the interface of theintermediate porosity layer 12M and the high porosity layer 12H.

Thereafter, similar to Embodiments 1 and 2, in the Si epitaxial growingdevice in which the annealing was carried out, epitaxial growth of Siwas carried out under the normal pressure for 17 minutes to form theepitaxial semiconductor film 13 made of single crystal Si having athickness of about 5 μm (FIG. 5A).

After the above step, similar to Embodiment 2, a support substrate madeof a PET sheet is bonded (not illustrated) and the peeling (FIG. 5B)etc. are carried out, thereby to obtain an intended thin filmsemiconductor 23. Also in this case, the peeling is carried out by thebreakage in the porous layer, that is, the breakage of the high porositylayer 12H or the vicinity thereof.

Also in this Embodiment 4, a surface layer which has a small porosity,that is, is denser, is formed. This surface layer becomes smoother bythe annealing in H₂. Therefore, the epitaxial semiconductor film 13epitaxially grown on this, that is, the thin film semiconductor 23formed by this, is formed as a semiconductor having more excellentcrystallinity.

In this Embodiment 4, the porous layer 12 was formed by incrementallyincreasing the current density in the formation of the intermediateporosity layer 12M beneath the surface 12S, so the porosity between thehigh porosity layer 12H and the surface layer 12S gradually changes, andtherefore the strain caused between the two layers is effectivelyrelieved, that is, buffered, by the intermediate porosity layer 12M.After performing the annealing in the H₂ atmosphere, a flatter andsmoother surface can be formed. Accordingly, the epitaxial semiconductorfilm formed on the surface layer and accordingly the finally obtainedthin film semiconductor can be formed as a thin film semiconductorhaving a more excellent crystallinity and higher reliability.

[Embodiment 5]

FIG. 8 is a process diagram of this embodiment. In this embodiment, inthe porous layer 12, the high porosity layer was formed in the bottom ofor beneath the intermediate porosity layer, that is, inside of thesubstrate 11 which was not made porous.

In this embodiment as well, similar to Embodiments 1 and 2, asemiconductor substrate 11 made of single crystal Si doped with boron Band having a resistivity of for example 0.01 to 0.02 Ωcm is prepared(FIG. 8A).

Then, with respect to this semiconductor substrate 11, similar toEmbodiments 1 and 2, an anodizing device of a double cell structureexplained referring to FIG. 1 was used. And an electrolytic solutionmade of HF:C₂ H₅ OH=1:1 was filled into both of the first and secondcells 1A and 1B. Then, a cur-rent was passed between the Pt electrodes3A and 3B immersed in the electrolytic solutions of the electrolyticsolution cells 1A and 1B by a DC current source 2.

In this Embodiment 5, similar to Embodiment 2, first, the current wassupplied for 8 minutes with a low current density of 1 mA/cm². By this,a surface layer 12S having a porosity of 16% and a thickness of about1.7 μm was formed (FIG. 8B). Then, similar to Embodiment 2, the currentsupply was once stopped and the anodization was carried out with acurrent supply of 7 mA/cm² for 8 minutes to form an intermediateporosity layer 12M having a porosity of 26% and a thickness of 6.3 μmbeneath the surface layer 12S (FIG. 8C).

Thereafter, the current supply was stopped once, then, in thisembodiment, a current with a high current density of 200 mA/cm² wasintermittently supplied. Namely, a current of 200 mA/cm² was suppliedfor 0.7 second at first, the current supply was stopped again for oneminute, and then a current of 200 mA/cm² was supplied for 0.7 second,the current supply was further stopped for one minute, and then acurrent of 200 mA/cm² was supplied for 0.7 second. Namely, theanodization was carried out by intermittently supplying current of ahigh current density three times. By this, a high porosity layer 12Hhaving a higher porosity in comparison with the intermediate porositylayer 12M, i.e., a porosity of about 60%, and a thickness of about 50 nmwas formed under the intermediate porosity layer 12M (FIG. 8D). In thisway, a porous layer 12 comprising the surface layer 12S, theintermediate porosity layer 12M, and the high porosity layer 12H isformed.

In the porous layer 12 formed in this way, the porosity between the highporosity layer 12H and the intermediate porosity layer 12M and furtherbetween the high porosity layer 12H and the substrate 11 greatlydiffers, so a large strain is applied at the interface and the vicinityof the interface and the strength at this location becomes extremelyweak.

After forming the porous layer 12 in this way, similar to theexplanation referring to Embodiment 2, annealing was carried out in a Siepitaxial growing device in an H₂ atmosphere under a normal pressure tomake the surface layer 12S of the porous layer 12 smooth andsimultaneously the high porosity layer 12H is made fragile.

Thereafter, similar to Embodiment 2, in the Si epitaxial growing devicein which the annealing was carried out, epitaxial growth of Si wascarried out under the normal pressure for 17 minutes to form anepitaxial semiconductor film 13 made of single crystal Si having athickness of about 5 μm (FIG. 8E).

Then, similar to the above embodiments, the epitaxial semiconductor film13 and the semiconductor substrate 11 are separated (FIG. 8F).

By the intermittent supply of a large current density, the high porositylayer 12H is formed in the bottom of or beneath the intermediateporosity layer 12M. And the high porosity layer 12H can be formed withan extremely high porosity. The porosity of the high porosity layer 12His remarkably improved by the annealing in the H2 atmosphere.Accordingly, the epitaxial semiconductor film 12 on the porous layer 12can be extremely easily peeled in the high porosity layer 12H or thevicinity thereof.

FIG. 23 and FIG. 24 are diagrammatical views based on micrographs of100,000 magnifications of the cross-section of the intermediate porositylayer 12M and the high porosity layer 12H in this embodiment before andafter the annealing in the H₂ atmosphere. As apparent from a comparisonof the two, the growth of the crystal grains is caused by the annealingin H₂. A remarkable expansion and growth of the pore portions occursparticularly in the high porosity layer 12H. An extremely rough layer ofa columnar form (FIG. 24 shows cross-section of part where there are nocolumns) is formed. The fragility in this part becomes remarkable.

Embodiment 5 shows a case where the high porosity layer 12H is formed atthe interface with the semiconductor substrate 11 by the intermittentsupply of a large current, but it is also possible to similarly form thehigh porosity layer 12H at the interface with the semiconductorsubstrate 11 by a means other than such an intermittent supply of alarge current. This case is shown in Embodiments 6 and 7 and Embodiment8.

[Embodiment 6]

In this embodiment, an explanation will be made by referring to theprocess diagram of FIG. 8. In this embodiment, the surface layer 12S andthe intermediate porosity layer were formed according to a processsimilar to that explained referring to Embodiment 2.

Namely, in this embodiment, similar to Embodiment 2, a wafer-likesemiconductor substrate 11 made of single crystal Si doped with boron Bat a high concentration and having a relative resistance of for example0.01 to 0.02 Ωcm was prepared (FIG. 8A).

Next, an anodizing device of a double cell structure explained referringto FIG. 1 was used. And an electrolytic solution made of HF:C₂ H₅ OH=1:1was filled into both of the first and second cells 1A and 1B. Then, acurrent was passed between the Pt electrodes 3A and 3B immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, the current was supplied for 8 minutes with a low current densityof 1 mA/cm². By this, a surface layer 12S similar to that in Embodiment2 is formed (FIG. 8B). The current supply was once stopped, then acurrent was supplied with a current density of 7 mA/cm² for 8 minutes toform an intermediate porosity layer 12M similar to that in Embodiment 2beneath the surface layer 12S (FIG. 8C). Further, the current supply wasstopped once, then, in this embodiment, a current of a so-called middlecurrent density of 60 mA/cm², lower than the large current supplied inEmbodiment 2, but higher than the supplied current at the formation ofthe surface layer 12S and the intermediate porosity layer 12M, wassupplied for 1.9 seconds. By this, similar to Embodiment 5, a highporosity layer 12H having a porosity of about 60% and a thickness ofabout 50 nm is formed at the interface with the surface of thesemiconductor substrate 11 under the intermediate porosity layer 12M(FIG. 8D). In this way, a porous layer 12 comprising the surface layer12S, the intermediate porosity layer 12M, and the high porosity layer12H is formed.

In the porous layer 12 formed in this way as well, the porosity greatlydiffers among the high porosity layer 12H, the intermediate porositylayer 12M, and the substrate 11, therefore a large strain is applied atthe interface and the vicinity of the interface, and thus the strengtharound here becomes extremely weak.

After the formation of the porous layer 12 in this way, similar to theexplanation referring to Embodiment 2, annealing is carried out in a Siepitaxial growing device in an H2 atmosphere under a normal pressure tomake the surface layer 12S of the porous layer 12 smooth andsimultaneously make the strength near the interface of the intermediateporosity layer 12M and the high porosity layer 12H fragile.

Thereafter, similar to Embodiment 2, in the Si epitaxial growing devicein which the annealing was carried out, epitaxial growth of Si iscarried out under the pressure for 17 minutes to form an epitaxialsemiconductor film 13 made of single crystal Si having a thickness ofabout 5 μm (FIG. 8E).

Then, similar to the above embodiments, the epitaxial semiconductor film13 is peeled from the semiconductor substrate 11 thereby to obtain anintended thin film semiconductor 23 (FIG. 8F).

[Embodiment 7]

In this embodiment, an explanation will be made by referring to theprocess diagram of FIG. 8. In this embodiment as well, similar toEmbodiment 2, a wafer-like semiconductor substrate 11 made of singlecrystal Si doped with boron B at a high concentration and having arelative resistance of for example 0.01 to 0.02 Ωcm was prepared (FIG.8A).

Also in this case, an anodizing device of a double cell structureexplained referring to FIG. 1 was used. And an electrolytic solutionmade of HF:C₂ H₅ OH=1:1 was filled into both of the first and secondcells 1A and 1B. Then, a current was passed between the Pt electrodes 3Aand 3B immersed in the electrolytic solutions of the electrolyticsolution cells 1A and 1B by a DC current source 2.

Then, first, in this embodiment, a current was supplied with a lowcurrent density of 1 mA/cm² for 6 minutes. By this, a surface layer 12Shaving a porosity of 16% and a thickness of 1.7 μm was formed (FIG. 8B).The current supply was stopped once, then current was supplied with acurrent density of 4 mA/cm² for 10 minutes. By this, the intermediateporosity layer 12M having a porosity of 22% and a thickness of 5.8 μm isformed beneath the surface layer 12S, that is, on the inner side fromthe surface layer 12S (FIG. 8C). Further, the current supply was stoppedonce, then, in this embodiment, a middle current of 60 mA/cm² wassupplied for 2 seconds. By this, the high porosity layer 12H having aporosity of about 60% and a thickness of about 50 nm is formed at theinterface with the semiconductor substrate 11 under the intermediateporosity layer 12M (FIG. 8D). In this way, a porous layer 12 comprisingthe surface layer 12S, the intermediate porosity layer 12M, and the highporosity layer 12H is formed.

In the porous layer 12 formed in this way, the porosity greatly differsamong the high porosity layer 12H, the intermediate porosity layer 12M,and the substrate 11, therefore a large strain is applied at theinterface and the vicinity of the interface, and thus the strengtharound here becomes extremely weak.

After forming the porous layer 12 in this way, similar to theexplanation referring to Embodiment 2, annealing is carried out in a Siepitaxial growing device in an H₂ atmosphere under a normal pressure tomake the surface layer 12S of the porous layer 12 smooth andsimultaneously make the strength near the interface of the intermediateporosity layer 12M and the high porosity layer 12H inside the porouslayer 12 fragile.

Thereafter, similar to Embodiment 2, in the Si epitaxial growing devicein which the annealing was carried out, epitaxial growth of Si iscarried out under the normal pressure for 17 minutes to form anepitaxial semiconductor film 13 made of single crystal Si having athickness of about 5 μm (FIG. 8E). The epitaxial semiconductor film 13is peeled from the semiconductor substrate 11 to obtain the intendedthin film semiconductor 23 (FIG. 8F).

[Embodiment 8]

In this embodiment, the explanation will be made by referring to theprocess diagram of FIG. 8. In this embodiment, similar to Embodiment 2,a wafer-like semiconductor substrate 11 made of single crystal Si dopedwith boron B at a high concentration and having a relative resistance offor example 0.01 to 0.02 Ωcm was prepared (FIG. 8A).

Then, in this case as well, anodization is carried out by using ananodizing device of a double cell structure explained referring to FIG.1, but in this embodiment, an electrolytic solution of HF:C₂ H₅ OH=1.2:1was filled into the first cell 1A, and an electrolytic solution of HF:C₂H₅ OH=1:1 was filled into the second cell 1B. Then, current was passedbetween the Pt electrodes 3A and 3B arranged immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, current was supplied with a low current density of 1 mA/cm² for 5minutes. By this, a surface layer 12S having a porosity of 13% and athickness of 1.5 μm was formed (FIG. 8B). The current supply was stoppedonce, then the current was supplied with a current density of 5 mA/cm2for 5 minutes. By this, an intermediate porosity layer 12M having aporosity of 18% and a thickness of 5 μm was formed under the surfacelayer 12S (FIG. 8C). Further, the current supply was once stopped, thena current with a middle current density of 80 mA/cm2 was supplied for 3seconds. By this, a high porosity layer 12H having a porosity of about60% and a thickness of about 50 nm was formed at the interface with thesemiconductor substrate 11 under the intermediate porosity layer 12Mwhich was not made porous (FIG. 8D). In this way, a porous layer 12comprising the surface layer 12S, the intermediate porosity layer 12M,and the high porosity layer 12H is formed.

In the porous layer 12 formed in this way, the porosity greatly differsamong the high porosity layer 12H, the intermediate porosity layer 12M,and the substrate 11, therefore a large strain is applied at theinterface and the vicinity of the interface, and thus the strengtharound here becomes extremely weak.

After the formation of the porous layer 12 in this way, similar to theexplanation referring to Embodiment 2, annealing is carried out in a Siepitaxial growing device in an H₂ atmosphere under a normal pressure tomake the surface layer 12S of the porous layer 12 smooth andsimultaneously make the strength near the interface of the intermediateporosity layer 12M and the high porosity layer 12H inside the porouslayer 12 fragile.

Thereafter, similar to Embodiment 2, in the Si epitaxial growing device,in which the annealing was carried out, epitaxial growth of Si wascarried out under the normal pressure for 17 minutes to form anepitaxial semiconductor film 13 made of single crystal Si having athickness of about 5 μm (FIG. 8E).

Then, in this case, the bonding of for example a PET sheet (notillustrated) and the peeling of the epitaxial semiconductor film 12 fromthe semiconductor substrate 11 are carried out to obtain the intendedthin film semiconductor 23 (FIG. 8F).

[Embodiment 9]

This embodiment is based on a process similar to Embodiment 2, but anoxidation step is added preceding the heat treatment with respect to theporous layer 12 in the H₂ atmosphere. This will be explained byreferring to FIGS. 4 and 5. In this embodiment, similar to Embodiment 2,a wafer-like semiconductor substrate 11 made of single crystal Si dopedwith boron B at a high concentration and having a relative resistance of0.01 to 0.02 Ωcm was prepared (FIG. 4A).

Further, an anodizing device of a double cell structure explainedreferring to FIG. 1 was used. And an electrolytic solution made of HF:C₂H₅ OH=1:1 was filled into both of the first and second cells 1A and 1B.Then, a current was passed between the Pt electrodes 3A and 3B immersedin the electrolytic solutions of the electrolytic solution cells 1A and1B by a DC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes to form a surface layer 12S (FIG. 4B). The current supply wasstopped once, then a current was supplied with a current density of 7mA/cm² for 8 minutes to form an intermediate porosity layer 12M underthe surface layer 12S (FIG. 4C). The current supply was stopped once,then a current of 200 mA/cm² was supplied for 3 seconds to form the highporosity layer 12H in the intermediate porosity layer 12M and therebyform a porous layer 12 comprised of the surface layer 12S, theintermediate porosity layer 12M, and the high porosity layer 12H (FIG.4D).

Thereafter, in this embodiment, the oxidation step is carried out. Thisoxidation was performed according to dry oxidation comprised of heatingin an oxygen atmosphere to 400° C. By this process, the internal portionof the porous layer 12 is oxidized, the occurrence of a large structuralchange in the porous layer is prevented even by the later heat treatmentin the H₂ atmosphere and the influence of the strain caused in thevicinity of the interface of the high porosity layer 12H exerted on thesurface layer 12S can be effectively avoided.

After this, by a similar method to that in Embodiment 2, the substrate11 is heat treated in an H₂ atmosphere by a Si epitaxial growing deviceunder a normal pressure, then the epitaxial growth of Si is carried out(FIG. 5A), and the adhesion, peeling, etc. of the support substrate byfor example a PET sheet are carried out, thereby to obtain the intendedthin film semiconductor 23 (FIG. 5B).

[Embodiment 10]

This embodiment shows a case where the concentration of the electrolyticsolution is changed in the anodization of the porous layer 12. In thiscase, the explanation will be made by referring to FIG. 4 and FIG. 5. Inthis embodiment, a wafer-like semiconductor substrate 11 made of singlecrystal Si doped with boron B at a high concentration and having arelative resistance of 0.01 to 0.02 Ωcm was prepared (FIG. 4A).

Then, anodization was carried out by using an anodizing device of adouble cell structure explained referring to FIG. 1, but in this case,an electrolytic solution of HF:C₂ H₅ OH=2:1 was filled into the firstcell 1A, an electrolytic solution of HF:C₂ H₅ OH=1:1 was filled into thesecond cell 1B, the Si substrate was sandwiched between theseelectrolytic solution cells, and a current was passed between the Ptelectrodes 3A and 3B disposed in the electrolytic solution tanks 1A and1B as the electrodes.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes. By this, a surface layer 12S having a porosity of 16% and athickness of 1.7 μm was formed (FIG. 4B). The current supply was stoppedonce, then a current was supplied at 7 mA/cm² for 8 minutes. By this, anintermediate porosity layer 12M having a porosity of 26% and a thicknessof 6.3 μm was formed (FIG. 4C).

Next, the concentration of the electrolytic solution of the first cell1A was changed to HF:C₂ H₅ OH=1:1. Then, the current density was raisedto 200 mA/cm², and current was supplied for 3 seconds. By this, a highporosity layer 12H having a porosity of about 60% and a thickness ofabout 0.5 μm was formed in the intermediate porosity layer 12M (FIG.4D). In this way, a porous layer 12 comprising the surface layer 12S,the intermediate porosity layer 12M, and the high porosity layer 12H isformed.

After this, similar to that in Embodiment 2, the substrate 11 isheat-treated in an H₂ atmosphere by a Si epitaxial growing device, thenthe epitaxial growth of Si is carried out (FIG. 5A), and the adhesion,peeling (FIG. 5B), etc. of the support substrate (not illustrated) byPET are carried out, thereby to obtain the thin film semiconductor 23.

In this embodiment, in the formation of the surface side of the porouslayer 12, that is, the surface layer 12S and the intermediate porositylayer 12M, the HF concentration is raised. When the HF concentration ofthe electrolytic solution is raised in this way, the porosity of theporous layer becomes small. Therefore, in this case, since a porouslayer having an extremely fine pore diameter is formed in the surfaceportion of the porous layer 12, the epitaxial semiconductor film to beepitaxially grown on this is formed as a film excellent incrystallinity.

Then, in this case, in the formation of the high porosity layer 12H, ifthe HF concentration of the electrolytic solution is high, a sufficientporosity may not be obtained with the current supply of a currentdensity of 200 mA/cm² for about 3 seconds, but in this embodiment, inthe production of the high porosity layer 12H, the HF concentration ofthe electrolytic solution is lowered, so a high porosity layer 12Hhaving a sufficiently high porosity can be produced.

[Embodiment 11]

This embodiment also shows a case where the concentration of theelectrolytic solution is changed in the anodization process. This willbe explained by referring to FIG. 6 and FIG. 7. In this embodiment, awafer-like semiconductor substrate 11 made of single crystal Si dopedwith boron B at a high concentration and having a relative resistance of0.01 to 0.02 Ωcm was prepared (FIG. 6A).

Then, an anodizing device of a double cell structure explained referringto FIG. 1 was used. And an electrolytic solution of HF:C₂ H₅ OH=2:1 wasfilled into the first cell 1A, an electrolytic solution of HF:C₂ H₅OH=1:1 was filled into the second cell 1B, the Si substrate wassandwiched between these electrolytic solution tanks 1A and 1B. Then, acurrent was passed between the Pt electrodes 3A and 3B disposed in theelectrolytic solution cells 1 A and 1 B as the electrodes.

First, the current was supplied with a current density of 1 mA/cm² for 8minutes. By this, a surface layer 12S having a porosity of about 14% anda thickness of about 2.0 μm was formed (FIG. 6B). The current supply wasstopped once, then a current was supplied at 7 mA/cm² for 6 minutes. Bythis, a first intermediate porosity layer 12M1 having a porosity ofabout 20% and a thickness of about 6.4 μm was formed (FIG. 6C).

Next, the concentration of the electrolytic solution of the first tank1A was changed to HF:C₂ H₅ OH=1:1. Then, a current was supplied again at7 mA/cm² for 2 minutes. By this, a second intermediate porosity layer12M2 having a porosity of about 26% and a thickness of about 1.7 μm wasformed (FIG. 6D).

Thereafter, the current supply was stopped once, and the concentrationof the electrolytic solution of the first tank 1A was further changed toHF:C₂ H₅ OH=1:1.5 to lower the concentration of the electrolyticsolution further. In this state, the current density was raised to 200mA/cm² and the current supplied for 2 seconds. By doing this, a highporosity layer 12H having a porosity of about 60% and thickness of about0.5 μm was formed in the second intermediate porosity layer 12M2 (FIG.6E). By this, a porous layer 12 comprising the surface layer 12S, theintermediate porosity layer 12M, and the high porosity layer 12H wasformed.

Thereafter, by a process similar to Embodiments 2 and 3, etc., thesubstrate 11 was heat-treated in an H₂ atmosphere by a Si epitaxialgrowing device under a normal pressure, then the epitaxial growth of Siis carried out to form the epitaxial semiconductor film 13 (FIG. 7A),and the epitaxial semiconductor film 13 is peeled from the semiconductorsubstrate 11 to obtain the intended thin film semiconductor 23 (FIG.7B). Also in this example, the porous layer adhered to the thin filmsemiconductor 23 was removed by etching.

In this embodiment, the first and second intermediate porosity layers12M1 and 12M2 were formed. In the step for forming the secondintermediate porosity layer 12M2, the concentration of the electrolyticsolution was lowered. The concentration of the electrolytic solution wasfurther lowered in the step for forming the high porosity layer 12H.Therefore, the porosity was stepwise raised from the surface layer 12Stoward the high porosity layer 12H, and therefore the influence of thestrain upon the surface of the porous layer 12 by the high porositylayer 12H can be effectively relieved, and the crystallinity of theepitaxial semiconductor film 13 to be epitaxially grown on the porouslayer 12 can be made higher.

Further, in the anodization of the high porosity layer 12H, theconcentration of the electrolytic solution was further lowered,therefore the fragility of this high porosity layer 12H can be furtherraised, and the separation, that is, the peeling property of theepitaxial semiconductor film 13 from the substrate 11, can be enhanced.

[Embodiment 12]

This embodiment also shows a case where the epitaxial semiconductorfilm, that is, the thin film semiconductor, has a multi-layer structure,i.e., p⁺ /p⁻ /n⁺ structure. FIG. 9 and FIG. 10 are process diagrams ofthis embodiment. In this embodiment, a wafer-like semiconductorsubstrate 11 made of single crystal Si doped with boron B at a highconcentration and having a relative resistance of for example 0.01 to0.02 Ωcm was prepared (FIG. 9A).

Next, an anodizing device of a double cell structure explained referringto FIG. 1 was used, an electrolytic solution of HF:C₂ H₅ OH=1:1 wasfilled into both of the first and second cells 1A and 1B, and a currentwas passed between the Pt electrodes 3A and 3B immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, a current was supplied at a current density of 1 mA/cm² for 8minutes to form a surface layer 12S (FIG. 9B). The current supply wasstopped once, then a current was supplied with a current density of 7mA/cm² for 8 minutes to form the intermediate porosity layer 12M (FIG.9C). Further, the current supply was stopped once, then a current of 200mA/cm² was supplied for 3 seconds. By performing this, a high porositylayer 12H was formed in the intermediate porosity layer 12M (FIG. 9D).In this way, a porous layer 12 comprised of the surface layer 12S, theintermediate porosity layer 12M, and the high porosity layer 12H isformed.

After forming the porous layer 12 in this way, similar to that explainedby referring to Embodiment 2, the annealing is carried out in a Siepitaxial growing device in an H₂ atmosphere under a normal pressure tomake the surface layer 12S of the porous layer 12 smooth and make thestrength near the interface of the intermediate porosity layer 12M andthe high porosity layer 12H fragile.

Thereafter, in the ordinary pressure Si epitaxial growing device inwhich the annealing was carried out, epitaxial growth, using SiH₄ gasand B₂ H₆ gas, was carried out for 2 minutes to form a first epitaxialsemiconductor layer 131 comprised of p⁺ Si doped with boron B at a highconcentration (FIG. 10A).

Next, the flow rate of the B₂ H₆ gas was changed and Si epitaxial growthwas carried out for 17 minutes to form a second epitaxial semiconductorlayer 132 made of p⁻ Si doped with boron at a low concentration (FIG.10B).

Thereafter, PH₃ gas is supplied in place of the B₂ H₆ gas and Siepitaxial growth doped with phosphorus to a high concentration iscarried out on the p⁻ epitaxial semiconductor layer 132 for 2 minutes toform a third epitaxial semiconductor film 133 made of n⁺ Si (FIG. 10C).In this way, an epitaxial semiconductor film 13 having a p⁺ /p⁻ /n⁺structure comprising the first to third epitaxial semiconductor layers131 to 133 is constituted.

After this, similar to the above embodiments, the peeling of theepitaxial semiconductor layer 13 from the substrate 11 and the otherprocess are carried out to obtain the intended thin film semiconductor23 (FIG. 10D). In this example, the porous layer adhered to the thinfilm semiconductor 23 was removed by etching. The thin filmsemiconductor 23 made of this p⁺ /p⁻ /n⁺ three-layer structure canconstitute a solar cell.

[Embodiment 13]

In this embodiment, instead of Si film in the process of Embodiment 12,the epitaxial semiconductor film 13 is formed as an epitaxialsemiconductor film by GaAs. Namely, in this case, in the steps of FIG.9A to FIG. 9D, steps similar to those in Embodiment 12 are employed, andthen, in the epitaxial growth of the epitaxial semiconductor film 13,hetero epitaxial growth is carried out at a substrate temperature of720° C. for 1 hour by an ordinary pressure MOCVD device by using TMGa(trimethyl gallium) and AsH₃ as the source material gas according to theMOCVD (Metal Organic Chemical Vapor Deposition) process, thereby, anepitaxial semiconductor film 13 made of GaAs having a film thickness ofabout 3 μm is formed.

Thereafter, the epitaxial semiconductor film 13 is peeled from thesemiconductor substrate 11 to obtain the thin film semiconductor 23 madeof the epitaxial semiconductor film 13.

[Embodiment 14]

This embodiment shows a case where a solar cell is produced. FIG. 11 toFIG. 14 are process diagrams thereof. In this embodiment, an epitaxialsemiconductor film made of a p⁺ /p⁻ /n⁺ three-layer structure is formedby a similar process to that for Embodiment 12. Namely, in thisembodiment, a wafer-like semiconductor substrate 11 made of singlecrystal Si doped with boron B at a high concentration and having arelative resistance of for example 0.01 to 0.02 Ωcm was prepared.

Then, in this case, an anodizing device of a double cell structureexplained referring to FIG. 1 was used, and an electrolytic solution ofHF:C₂ H₅ OH=1:1 was filled into both of the first and second cells 1Aand 1B, and a current was passed between the Pt electrodes 3A and 3Bimmersed in the electrolytic solution of the electrolytic solution cells1A and 1B by a DC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minute to form a surface layer 12S (FIG. 11A). The current supply wasstopped once, then a current was supplied with a current density of 7mA/cm² for 8 minutes to form an intermediate porosity layer 12M (FIG.11B). Further, the current supply was stopped once, then a current of200 mA/cm² was supplied for 3 seconds. By this, a high porosity layer12H was formed in the intermediate porosity layer 12M (FIG. 11C). Inthis way, a porous layer 12 comprised of the surface layer 12S, theintermediate porosity layer 12M, and the high porosity layer 12H isformed.

After forming this porous layer 12, annealing in an H₂ atmosphere iscarried out in a Si epitaxially growing device by a process similar tothat explained referring to Embodiment 2. When performing this, thesurface layer 12S of the porous layer 12 is made smooth and the strengthnear the interface of the intermediate porosity layer 12M and the highporosity layer 12H is weakened.

Thereafter, in the Si epitaxial growing device in which the annealingwas carried out, epitaxial growth using SiH₄ gas and B₂ H₆ gas wascarried out for 2 minutes to form a first epitaxial semiconductor layer131 made of p⁺ Si having a thickness of 0.5 μm and doped with boron B to10¹⁹ atoms/cm³. Next, the flow rate of the B₂ H₆ gas was changed and theSi epitaxial growth was carried out for 17 minutes to form a secondepitaxial semiconductor layer 132 made of p⁻ Si having a thickness of 5μm and doped with boron B to 10¹⁶ atoms/cm³. Further, PH₃ gas wassupplied in place of the B₂ H₆ gas and epitaxial growth was carried outfor 2 minutes to form a third epitaxial semiconductor layer 133 made ofn⁺ Si doped with phosphorus P to a high concentration of 10¹⁹ atoms/cm³.By this, an epitaxial semiconductor film 13 having a p⁺ /p⁻ /n⁺structure comprising the first to third epitaxial semiconductor layers131 to 133 was formed (FIG. 12A).

Next, in this embodiment, an SiO₂ film, that is, a transparentinsulating film 16, is formed on the epitaxial semiconductor film 13 bysurface thermal oxidation and then is patterned by photolithography toform openings 16W for contact with an electrode or interconnections(FIG. 12B). The openings 16W can be formed in a parallel array ofstripes extending in a direction orthogonal to the sheet surface in thefigure while maintaining a required interval between them. It ispossible to reduce the production of carriers at the interface andrecombination to a maximum extent by the SiO₂ film formed in this way.

Next, a metal film is vapor deposited over the entire surface andpattern etching is carried out by photolithography to form theelectrodes or interconnections 17 on the light receiving surface sidealong the stripe-like openings 16W (FIG. 13A). This metal film as theelectrodes or interconnections 17 can be constituted by a multilayerfilm formed by successively vapor depositing for example a Ti filmhaving a thickness of 30 nm, Pd having a thickness of 50 nm, and Ag of athickness of 100 nm and further applying Ag plating on this. Thereafter,annealing is carried out for 20 to 30 minutes at 400° C.

On the other hand, a flexible printed circuit board 20 comprised of thetransparent substrate 18 made of, for example, a flexible plastic sheet,on which interconnections 19 of the required circuit are formed, isprepared. This printed circuit board 20 is laid on the epitaxialsemiconductor film 13 on which the insulating film 16 is formed, andthey are bonded by a binder 21 which is transparent and is insulative.At this time, the interconnections 19 and the electrodes orinterconnections 17, which should be connected to each other, are madeto face each other and a solder is interposed between them so as to formthe electrical connection (FIG. 13B). At this time, as the binder 21,one having a slightly stronger strength than the separation strength ofthe porous layer is used.

Next, an external force for separating the semiconductor substrate 11from the printed circuit board 20 is given. When performing this, thesemiconductor substrate 11 and the epitaxial semiconductor film 13 areseparated at the fragile high porosity layer 12H of the porous layer 12or the vicinity thereof. As a result, a thin film semiconductor 23,having the epitaxial semiconductor film 13, is obtained on the printedcircuit board 20 (FIG. 14A).

In this case, the porous layer 12 remains on the back surface of thethin film semiconductor 23. Silver paste is coated on this back surfaceand a metal sheet is further bonded on the silver paste to constituteanother back surface electrode 24. In this way, a solar cell having athin film semiconductor 23 having a p⁺ /p⁻ /n⁺ structure is constitutedon the printed circuit board 20 (FIG. 14B). In this case, the metalelectrode 24 acts also as a film protecting the device layer at the backsurface of the solar cell.

Note that, the above Embodiment 14 shows a case where a solar cell whichcan be similarly made flexible is integrally formed on a flexibleprinted circuit board, but it is also possible to adopt a structurewhere the solar cell is integrally formed on a rigid substrate such as aglass substrate.

Next, embodiments of the process for producing a thin film semiconductoror a solar cell, where the anodization conditions for formingparticularly the high porosity layer of the porous layer serving as theseparation layer are changed, will be explained.

[Embodiment 15]

The explanation will be made by referring to the process diagrams ofFIG. 15 and FIG. 16. In this case, similar to Embodiment 6, asemiconductor substrate 11 made of single crystal Si doped with boron Band having a resistivity of for example 0.01 to 0.02 Ωcm is prepared(FIG. 15A).

For forming the porous layer in this semiconductor substrate 11, ananodizing device of a double cell structure explained referring to FIG.1 was used and an electrolytic solution made of HF:C₂ H₅ OH=1:1 wasfilled into both of the first and second cells 1A and 1B. Then, acurrent was passed between the Pt electrodes 3A and 3B immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes. In this way, a surface layer 12S of a low porosity was formed(FIG. 15B). The current supply was stopped once, then a current wassupplied with 7 mA/cm² for 8 minutes. In this way, an intermediateporosity layer 12M was formed (FIG. 15C). Further, the current supplywas stopped once, then, in this embodiment, a current was supplied at 90mA/cm² for 5 seconds. By this, a high porosity layer 12H was formed inthe intermediate porosity layer 12M (FIG. 15D). After this, a currentwas supplied at 7 mA/cm² for 8 minutes. In this way, a porous layer 12comprising the surface layer 12S, the intermediate porosity layer 12M,and the high porosity layer 12H is formed.

Thereafter, annealing similar to that in Embodiment 2 was carried outand epitaxial growth of Si was carried out for 17 minutes on the porouslayer 12 to form an epitaxial semiconductor film 13 made of singlecrystal Si having a thickness of about 5 μm (FIG. 16A).

Then, an external force was given in a direction for separating theepitaxial semiconductor film 13 from the semiconductor substrate 11.When performing this, the epitaxial semiconductor film 13 is separatedat the high porosity layer 12H or in the vicinity thereof, and the thinfilm semiconductor 23 is obtained (FIG. 16B).

[Embodiment 16]

The explanation will be made by referring to the process diagram of FIG.17. In this case, similar to Embodiment 6, a semiconductor substrate 11made of single crystal Si doped with boron B and having a resistivity offor example 0.01 to 0.02 Ωcm is prepared (FIG. 17A).

For forming the porous layer in this semiconductor substrate 11, ananodizing device of a double cell structure explained referring to FIG.1 was used and an electrolytic solution made of HF:C₂ H₅ OH=1:1 wasfilled into both of the first and second cells 1A and 1B. Then, acurrent was passed between the Pt electrodes 3A and 3B immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes. In this way, a surface layer 12S of a low porosity was formed(FIG. 17B). The current supply was stopped once, then a current wassupplied at 7 mA/cm² for 8 minutes. In this way, an inter-mediateporosity layer 12M was formed (FIG. 17C). Further, the current supplywas stopped once, then, in this embodiment, a current was supplied at 30mA/cm² for 15 seconds. By this, a high porosity layer 12H was producedunder the intermediate porosity layer 12M (FIG. 17D). After this, acurrent was supplied at 7 mA/cm² for 8 minutes. In this way, a porouslayer 12 comprising the surface layer 12S, the intermediate porositylayer 12M, and the high porosity layer 12H is formed.

Thereafter, annealing similar to that in Embodiment 2 was carried out.Then, epitaxial growth of Si was carried out for 17 minutes on theporous layer 12 to form an epitaxial semiconductor film 13 made ofsingle crystal Si having a thickness of about 5 μm (FIG. 17E).

Then, an external force was given in the direction for separating theepitaxial semiconductor film 13 from the semiconductor substrate 11. Inthis case, however, there were some cases where the epitaxialsemiconductor film 13 could not always be separated from thesemiconductor substrate 11 well.

[Embodiment 17]

The explanation will be made by referring to the process diagram of FIG.18. In this case, similar to Embodiment 6, a semiconductor substrate 11made of single crystal Si doped with boron B and having a resistivity offor example 0.01 to 0.02 Ωcm is prepared (FIG. 18A).

For forming the porous layer in this semiconductor substrate 11, ananodizing device of a double cell structure explained referring to FIG.1 was used and an electrolytic solution made of HF:C₂ H₅ OH=1:1 wasfilled into both of the first and second cells 1A and 1B. Then, acurrent was passed between the Pt electrodes 3A and 3B immersed in theelectrolytic solutions of the electrolytic solution cells 1A and 1B by aDC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes. In this way, a surface layer 12S of low porosity was formed(FIG. 18B). The current supply was stopped once, then a current wassupplied with 7 mA/cm² for 8 minutes. In this way, an intermediateporosity layer 12M was formed (FIG. 18C). Further, the current supplywas stopped once, then, in this embodiment, a current was supplied at 80mA/cm² for 5 seconds. By performing this, a high porosity layer 12H wasrespectively produced in the intermediate porosity layer 12M and beneaththe intermediate porosity layer 12M, that is, at the interface with thesemiconductor substrate 11 (FIG. 18D). After this, a current wassupplied at 7 mA/cm² for 8 minutes. In this way, a porous layer 12comprising the surface layer 12S, the intermediate porosity layer 12M,the high porosity layer 12H, the intermediate porosity layer 12M, andthe high porosity layer 12H is formed.

Thereafter, annealing similar to that in Embodiment 2 was carried out.Epitaxial growth of Si was carried out for 17 minutes on the porouslayer 12 to form an epitaxial semiconductor film 13 made of singlecrystal Si having a thickness of about 5 μm (FIG. 18E).

Then, an external force was given in a direction for separating theepitaxial semiconductor film 13 from the semiconductor substrate 11.When performing this, the epitaxial semiconductor film 13 is separatedat either high porosity layer 12H of the porous layer 12, and a thinfilm semiconductor made of the epitaxial semiconductor film 13 isobtained.

As explained above, as apparent from Embodiment 6 (FIG. 8), Embodiment15 (FIG. 15 and FIG. 16), Embodiment 16 (FIG. 17), Embodiment 17 (FIG.18), and further Embodiment 5 (FIG. 8), in the step for forming the highporosity layer 12H, the position of the high porosity layer 12H changesaccording to the amount of current supply for the anodization andfurther the method of supply of the cur-rent. For example, in a casewhere an electrolytic solution of the anodization of HF:C₂ H₅ OH=1:1 isused, if the current density is set to about 40 to 70 mA/cm², by theselection of the current supply time, the high porosity layer 12H can beformed in the lowermost layer of the porous layer, that is, at theinterface of the semiconductor substrate 11 where the porous layer isnot produced, and in the high current range of from 90 mA/cm² to forexample about 300 mA/cm², it could be formed in the intermediateporosity layer 12M. Then, also in the high current range, when thiscurrent supply is made intermittent for a short time, it can be formedin the intermediate porosity layer 12M. It was confirmed that theposition this high porosity layer could be selected according to designwith a good reproducibility.

Further embodiments of the process for producing the solar cellaccording to the present invention will be explained.

[Embodiment 18]

In this embodiment, it is made easy to lead out the terminals from theelectrodes on the light receiving side, that is, lead out theconductors. This will be explained by referring to FIG. 11, FIG. 12,FIG. 13 and FIG. 19. In this embodiment, similar steps as thoseexplained by referring to FIGS. 11A to 11C, FIGS. 12A and 12B, and FIG.13A of Embodiment 14 are employed. Further, in this embodiment, anepitaxial semiconductor film made of a p⁺ /p⁻ /n⁺ three-layer structureis formed by a similar process as that for Embodiment 12. Namely, awafer-like semiconductor substrate 11 made of single crystal Si dopedwith boron B at a high concentration and having a relative resistance offor example 0.01 to 0.02 Ωcm was prepared.

Next, in this case, an anodizing device of a double cell structureexplained referring to FIG. 1 was used and an electrolytic solution madeof HF:C₂ H₅ OH=1:1 was filled into both of the first and second cells 1Aand 1B. Then, a current was passed between the Pt electrodes 3A and 3Bimmersed in the electrolytic solutions of the electrolytic solutioncells 1A and 1B by a DC current source 2.

First, a current was supplied with a current density of 1 mA/cm² for 8minutes to form a surface layer 12S (FIG. 11A). The current supply wasstopped once, then a current was supplied with a current density of 7mA/cm² for 8 minutes to form an intermediate porosity layer 12M (FIG.11B). Further, the current supply was stopped once, then a current of200 mA/cm² was supplied for 3 seconds. By performing this, a highporosity layer 12H was formed in the intermediate porosity layer 12M(FIG. 11C). In this way, a porous layer 12 comprised of the surfacelayer 12S, the intermediate porosity layer 12M, and the high porositylayer 12H is formed.

After forming this porous layer 12, annealing is carried out in an H₂atmosphere in a Si epitaxially growing device by a process similar tothat explained by referring to Embodiment 2. When performing this, thesurface layer 12S is made smooth and the strength near the interface ofthe intermediate porosity layer 12M and the high porosity layer 12H isweakened.

Thereafter, in the Si epitaxial growing device in which the annealingwas carried out, epitaxial growth using SiH₄ gas and B₂ H₆ gas wascarried out under the normal pressure for 2 minutes to form a firstepitaxial semiconductor layer 131 made of p⁺ Si having a thickness of0.5 μm and doped with boron B to 10¹⁹ atoms/cm³. Next, the flow rate ofthe B₂ H₆ gas was changed and the Si epitaxial growth was carried outfor 17 minutes to form a second epitaxial semiconductor layer 132 madeof p⁻ Si having a thickness of 5 μm and doped with boron B to 10¹⁶atoms/cm³. Further, PH₃ gas was supplied in place of the B₂ H₆ gas andthe epitaxial growth was carried out for 2 minutes to form a thirdepitaxial semiconductor layer 133 made by n⁺ Si doped with phosphorus Pto a high concentration of 10¹⁹ atoms/cm³. An epitaxial semiconductorfilm 13 having a p⁺ /p⁻ /n⁺ structure comprising the first to thirdepitaxial semiconductor layers 131 to 133 was formed as a result (FIG.12A).

Next, in this embodiment, an SiO₂ film, that is, a transparentinsulating film 16, was formed on the epitaxial semiconductor film 13 bysurface thermal oxidation and was patterned by photolithography to formopenings 16W for contact with the electrodes or interconnections (FIG.12B). The openings 16W can be formed in a parallel array of stripesextending in a direction orthogonal to the sheet surface in the figurewhile maintaining a required interval between them. It is possible toreduce the production of carriers at the interface and recombination toa maximum extent by the SiO₂ film formed in this way.

Next, a metal film is vapor deposited over the entire surface andpattern etching is carried out by photolithography to form theelectrodes or interconnections 17 on the light receiving surface sidealong the stripe-like openings 16W (FIG. 13A, FIG. 19A). This metal filmforming the electrodes or interconnections 17 can be constituted by amulti-layer film formed by successively vapor depositing for example aTi film having a thickness of 30 nm, Pd having a thickness of 50 nm, andAg of a thickness of 100 nm and further applying Ag plating on this.Thereafter, annealing is carried out for 20 to 30 minutes at 400° C.

Next, in this embodiment, on the stripe-like electrodes orinterconnections 17, conductors 41, i.e., in this embodiment, metalwires, are respectively bonded along them. A transparent substrate 42 isbonded onto this by a transparent binder 21 (FIG. 19B). The bonding ofthe conductors 41 to the electrodes or interconnections 17 can be bysoldering. One or the other ends of these conductors 41 are respectivelymade longer than the electrodes or interconnections 17 and led outward.

Thereafter, an external force is given for separating the semiconductorsubstrate 11 and the transparent substrate 42 from each other. Whenperforming this, the semiconductor substrate 11 and the epitaxialsemiconductor film 13 are separated at the fragile high porosity layer12H or the vicinity thereof, and a thin film semiconductor 23, to whichthe epitaxial semiconductor film 13 is bonded, is obtained on thetransparent substrate 42 (FIG. 20A).

In this case, the porous layer 12 remains on the back surface of thethin film semiconductor 23. Silver paste is coated on this back surfaceand a sheet of metal is further bonded with it to constitute anotherback surface electrode 24. In this way, a solar cell having a thin filmsemiconductor 23 having a p⁺ /p⁻ /n⁺ structure is constituted on theprinted circuit board 20 (FIG. 20B). This metal electrode 24 acts alsoas a film protecting the device layer on the back surface of the solarcell.

In the solar cell formed in this way, despite the fact that the lightreceiving side electrodes or interconnections 17 are covered by thetransparent substrate 42, since the conductors 41 are used for theelectrical lead out to the outside, electrical connection to the outsideis easy. Further, since, for example, as in the above embodiments, theconductors 41 are led out from a plurality of electrodes orinterconnections 17 respectively brought into contact with the epitaxialsemiconductor film 13, that is, the active portion of the solar cell,the number of serially connected resistors of the solar cell can bereduced.

Further, since the conductors 41 are led to the outside in this way,when a plurality of solar cells are connected to each other, they can beeasily connected. Next, an explanation will be made of an embodimentwhere a plurality of solar cells are connected to each other andarranged on a common substrate.

[Embodiment 19]

Process diagrams of this embodiment are given in FIG. 21 and FIG. 22,but the steps up to FIG. 19B of Embodiment 18 are the same as those ofEmbodiment 19, so an explanation of the steps up to this overlappingthat of Embodiment 18 will be omitted. In FIG. 21 and FIG. 22, partscorresponding to those in FIG. 19 and FIG. 20 are given the samereferences and overlapping explanations are omitted.

In this embodiment, a plurality of semiconductor substrates 11 similarto that shown in FIG. 19B, that is, substrates where the porous layer 12is formed on the surface, an epitaxial semiconductor film 13 having a p⁺/p⁻ /n⁺ structure is formed on this, the electrodes or interconnections17 are brought into contact with predetermined portions of this, andconductors 41 are bonded to this are prepared. These are bonded to acommon transparent substrate 42 by a transparent binder 21. In this caseas well, the end portions of a plurality of conductors 41 are led fromthe semiconductor substrates 11 to the outside (FIG. 21A).

Thereafter, an external force is given in a direction for separating thesemiconductor substrates 11 and the common transparent substrate 42 fromeach other. When performing this, the semiconductor substrate 11 and theepitaxial semiconductor film 13 are separated at the fragile highporosity layer 12H or the vicinity thereof and thin film semiconductors23 made by epitaxial semiconductor films 13 are arranged on the commontransparent substrate 42 (FIG. 21B).

The porous layer 12 remains on each back surface of these thin filmsemiconductors 23. Silver paste is coated on each back surface and ametal sheet is further bonded to constitute another back surfaceelectrode 24. In this way, the common transparent substrate 42 hasarranged on it a plurality of solar cell elements 3 in each of which theactive portion of the solar cell is formed by a thin film semiconductor23 having a p⁺ /p⁻ /n⁺ structure, a light receiving surface sideelectrode or interconnections 17 are formed, and an electrode 24 isformed on the back surface (FIG. 21C).

Then, one end of a conductor 41 is soldered to each of the requiredelectrodes 24 and an insulating material 43 such as resin is filledbetween the solar cell elements to insulate them from each other (FIG.22A). In this case, the free ends of the conductors 41 on the lightreceiving surface side of the solar cell elements S which should beconnected to each other are led to the outside of the insulatingmaterial 43, and the free ends are connected to the back surfaceelectrodes 24 of for example adjoining solar cell elements S bysoldering etc.

The free ends of the conductors 41, of the first stage and the laststage of the plurality of solar cell elements 3 connected to each other,are led to the outside, then a protective insulating layer 44 is appliedby molding etc. in a manner to expose the transparent substrate 42 sideand to cover the solar cell elements S. In this way, a solar cell inwhich a plurality of solar cell elements S are arranged on a commontransparent substrate 42 and connected to each other in series isconstituted (FIG. 22B). It goes without saying that the incident lightsuch as sunlight strikes this solar cell from the transparent substrate42 side.

Note that, in the above examples, the conductor 41 is not limited to ametal wire. It is also possible to constitute the same by for example ametal strip etc.

Further, it is possible to constitute the transparent substrate 42 by arigid substrate such as a glass substrate or constitute the same by aflexible substrate made sheet. Where it is constituted by a flexiblesubstrate in this way, the entire solar cell can be constitutedflexible.

When producing a solar cell in this way, regardless of the fact that atransparent substrate is arranged on the light receiving surface, theconductors can be led out from the electrodes 17, so it is possible toreduce the number of serially connected resistors. Also, the conductorsare connected in a state where they are formed on the semiconductorsubstrate 11 and are mechanically strong and stable before theseparation of the thin film solar cell, therefore the connection can bereliably and easily carried out on a mass production basis. Further, aplurality of solar cells can be connected to each other easily byleading out the conductors in this way.

In FIG. 21 and FIG. 22, only two solar cell elements S were shown, butit goes without saying that more than two solar cell elements can bearranged and connected.

Further, in a solar cell, where the porous layer 12 remains on the backsurface of the thin film semiconductor, the porous layer 12 also has ahigh impurity concentration when the semiconductor substrate 11 has ahigh impurity concentration. Thus there is sometimes the inconveniencethat this porous layer 12 will absorb the light. In this case, thisporous layer 12 can be removed by for example etching. Next, anexplanation will be made of an embodiment of the process for producing alight emitting diode according to the present invention.

[Embodiment 20]

The explanation will be made by referring to FIG. 25 to FIG. 28. In thisembodiment, a p-type Si single crystal semiconductor substrate 11 wasprepared (FIG. 25A). The n-type impurity phosphorus was diffused in onemain surface thereof to form an retype semiconductor layer 101 (FIG.25B).

Using the anodizing device of FIG. 1, under irradiation of light, acurrent of 50 mA/cm² was supplied for 30 minutes to perform theanodization and thereby form a first high porosity layer 12H1 having arelatively high porosity in the surface of the semiconductor layer 101(FIG. 25C). Next, without the irradiation of light, anodization wasperformed by supplying a current of 7 mA/cm² for 10 minutes to form anintermediate porosity layer 12M to a depth traversing the semiconductorlayer 101 (FIG. 25D). Next, similarly without irradiating light,anodization was carried out at 200 mA/cm² for 7 seconds to form a secondhigh porosity layer 12H2 acting as the separation layer in theintermediate porosity layer 12M (FIG. 25E).

On the high porosity layer 12H1 of the surface, stripe-like electrodes102 extending in a direction orthogonal to the sheet surface in forexample FIG. 26 were arranged in parallel by for example vapordeposition of Au (FIG. 26A) and photolithographic patterning and etchingto provide stripe-like electrodes 102. Other methods for forming andattaching electrodes known to those skilled in this art may also beused. A transparent binder 103 was coated on the surface of thesubstrate 11 on which the electrodes 102 were formed (FIG. 26B) and atransparent substrate 104 was adhered on top (FIG. 26C).

Next, using the second high porosity layer 12H2 as the separation layer,the front surface side, to which the transparent substrate 104 of thesemiconductor substrate 11 was bonded, was separated from the substrate11 to constitute the light emitting diode substrate 111 (FIG. 26D). Thesubstrate 111 constituted in this way has a p-n junction comprised of ap-type semiconductor layer 105 formed by the intermediate porosity layer12M and a p-type high porosity layer 12H1 formed on the surface of then-type semiconductor layer 101 formed on this.

On the back surface (separation surface) of the substrate 11, similarly,stripe-like back surface electrodes 106 comprised for example of an Auvapor deposited layer are formed facing the stripe-like electrodes 102(FIG. 27A). A transparent binder 103 is coated on the electrode106-forming surface of the substrate 111 (FIG. 27B) and a transparentsubstrate 104 is bonded (FIG. 27C). The substrate 111 is divided forevery pair of electrodes 102 and 106 (FIG. 28A) to obtain the intendedlight emitting diodes 107 (FIG. 28B). The light emitting diode, i.e.,so-called EL, constituted in this way, emits light as indicated by anarrow in FIG. 28B. The main light emitting portion becomes the highporosity layer 12H1 and has a high light emitting efficiency. This isbecause the super lattice structure is constituted by the high porositylayer 12H1, the active surface of which being formed sufficiently thin.The above embodiment shows a case where the semiconductor layer 101 isformed by the diffusion of impurities, but it is also possible to formthis by ion implantation of the impurities, or by an epitaxially grownsemiconductor layer, solid phase growth, CVD (Chemical VaporDeposition), etc. Further, the semiconductor layer 101 is not alwaysformed on entire surface. It can be formed at predetermined parts byselective diffusion, ion implantation, etc. as well. Further, thesemiconductor substrate 11 can be constituted as an n-type too, and thelight emitting efficiency can be enhanced by using a high resistancesubstrate. Further, when the substrate 111 is thermally oxidized in anoxygen atmosphere and then separated, a blue-shift of the emitted lightwavelength can be obtained.

Note that, the above examples showed a case where the semiconductor film3 was peeled from the semiconductor substrate 11 by giving an externalforce for separating them from each other, but it is also possible topeel the semiconductor film 3 by supersonic wave vibration (ultrasonicirradiation) in certain cases.

In the above examples, in the anodization, peeling of the semiconductorfilm from the substrate may occur due to the supply of a larger current,the long time current supply, etc. This Si waste is sometimes adhered tothe electrolytic liquid cell. In this case, by taking the substrate 11out and then filling fluoronitric acid into the cell in place of theelectrolytic solution, the unnecessary semiconductor waste such as Sican be removed by etching. Further, the device for performing theanodization is not limited to the example of FIG. 2. A device immersingthe semiconductor substrate in a single cell can also be used.

According to the process the present invention, the porous layer isformed in the surface of the semiconductor substrate and a semiconductoris epitaxially grown on this and then peeled, therefore thesemiconductor substrate is consumed by only the thickness of the porousportion. After the formation and peeling of the epitaxial semiconductorfilm, it is possible to grind the surface of the semiconductor substrateand repeat the formation of the porous layer, the formation of theepitaxial semiconductor film, and the peeling. Therefore, repeated useof the semiconductor substrate is possible, and therefore thesemiconductor film can be cheaply produced. Further, when thesemiconductor substrate becomes thin due to the repeated use, thissemiconductor substrate per se can be used as the thin filmsemiconductor, for example, used for production of a solar cell.Accordingly, the semiconductor substrate never becomes useless and canbe used with almost no waste. This also makes it possible to reduce thecosts.

Further, by epitaxially growing on a semiconductor substrate, reduced inthickness by production of a thin film semiconductor or solar cell, asemiconductor, having a thickness corresponding to this reduction ofthickness so as to repeatedly produce a thin film solar cell, permanentuse of the same semiconductor substrate becomes possible and thereforesolar cells can be produced at a further lower cost and lower energy.

Further, according to the process of the present invention, a method inwhich a support substrate such as a printed circuit board is bonded ontothe epitaxial semiconductor film, the substrate and the epitaxialsemiconductor film are integrally joined, and then the support substrateis peeled from the semiconductor substrate together with the epitaxialsemiconductor film, therefore the type of this substrate is not limitedand a thin film single crystal or solar cell can be formed on asubstrate which never was able to be considered by the conventionalmethod in semiconductor technology, for example, metal plate, ceramic,glass, and resin.

Further, where the method of merely epitaxially growing thesemiconductor layer on a porous layer, having a single porosity, isadopted, it is necessary to reduce the porosity of the porous layerwhich becomes the seeds of the crystal growth so as to enhance thecrystallinity of the semiconductor film, therefore, at anodization, itis necessary to lower the current density and raise the HF mixing ratioof the electrolytic solution. However, when the porosity is lowered inthis way, the porous layer becomes hard, and the separation of theepitaxial semiconductor film becomes difficult. Therefore, when thecurrent density is raised in the anodization and the HF mixing ratio ofthe electrolytic solution is reduced so as to raise the porosity forweakening the separation strength, the separation becomes easier, butthe crystallinity of the epitaxial semiconductor film becomes extremelypoor. However, as mentioned before, by forming a porous layer having twonatures, i.e., where the porosity of the surface part of the porouslayer is made smaller and the porosity of the internal portion of theporous layer is larger, the epitaxial semiconductor film can be formedwell on the porous layer and in addition the epitaxial semiconductorfilm can be easily separated. For example, it is also possible to form aporous layer which is so weak that it can be easily separated by forexample a supersonic wave.

Further, in the high porosity layer to be formed as the a part of porouslayer, the larger the porosity, the easier the peeling, but the strainis large and the influence thereof reaches up to the surface layer ofthe porous layer. For this reason, cracking sometimes occurs in thesurface layer. Further, when performing the epitaxial growth, it becomesa cause inducing defects in the epitaxial semiconductor film. Contraryto this, as mentioned before, by forming an intermediate porosity layerhaving a slightly higher porosity than that of the surface layer betweenthe layer having a very high porosity and the surface layer having a lowporosity as a buffer layer for relieving the strain generated from theselayers, an epitaxial semiconductor film which is easy to peel and has agood quality can be formed.

Further, according to the present invention, in the anodization with ahigh current density, by intermittently passing the current, the highporosity layer 12H can be formed in the porous layer at the bottom ofthe porous layer or the vicinity thereof, therefore the surface layerand the high porous layer, acting as the peeling layer, can be spacedapart to the highest limit, and therefore the buffer layer can be madethin, the thickness of the porous layer is reduced by that amount, andthe consumption of the semiconductor substrate can be reduced, thus itbecomes possible to further lower the costs.

Further, in the process of the present invention, in the anodizationwith a low current density, when the porosity of the buffer layer, to beformed between the surface layer and peeling layer, is graduallyincreased toward the internal portion by gradually increasing thecurrent, the function of the buffer layer can be further enhanced.

Further, by performing the anodization in an electrolytic solutioncontaining a mixture of hydrogen fluoride and ethanol or hydrogenfluoride and methanol, the porous layer can be easily formed. In thiscase, by changing the composition of this electrolytic solution alsowhen changing the current density of the anodization, the range ofadjustment of porosity becomes further larger.

Further, generation of unevenness at the surface of the porous layerbecomes conspicuous due to irradiation of light during the anodizationand the crystallinity of the epitaxial semiconductor film becomes poor,but in the present invention, by performing the anodization in a darkplace, this unevenness can be reduced or avoided and an epitaxialsemiconductor film having a good crystallinity can be formed.

Further, by heating the porous layer in a hydrogen gas atmosphere afterformation, the surface of the surface layer of the porous layer becomessmooth and thus an epitaxial semiconductor film having a goodcrystallinity could be formed. Further, by thermally oxidizing theporous layer after formation and before the heating step in the hydrogengas atmosphere, the internal portion of the porous layer is oxidized andtherefore even next annealing in hydrogen, it becomes hard to cause alarge structural change in the porous layer and it becomes hard for thestrain to transfer from the internal portion to the surface of theporous layer, and therefore an epitaxial semiconductor film having agood crystallinity can be formed.

Further, by using single crystal silicon as the semiconductor substrate,a single crystal silicon thin plate to be used in a solar cell can beproduced. Further, the semiconductor substrate doped with boron at ahigh concentration is made porous at the anodization while maintainingthe crystal state, therefore a good quality epitaxial semiconductor filmcan be formed.

Further, according to the process the present invention, two or moresemiconductor layers may be epitaxially grown on the surface of theporous layer and for example solar cells etc. can be easily produced.

Further, in the case where for example a solar cell is produced, byforming an insulating film on the surface of this multi-layer epitaxialsemiconductor film and further forming an electrode on this, the currentcan be led out from the epitaxial semiconductor film while reducing thegeneration of carriers and recombination at the interface with theepitaxial semiconductor film as much as possible.

Further, according to the process of the present invention, by bonding atransparent printed circuit board to the electrode surface of the solarcell, a substrate on which the interconnections of the circuit for thesolar cell are formed and the solar cell can be integrally joined andthe combination of a printed circuit board and thin film single crystalsolar cell, which could never have been thought of in the field ofsemiconductor technology in past, can be easily achieved.

Further, in the solar cell produced according to the present invention,for example the single crystal Si can be formed thin, that is, flexibleas the epitaxial semiconductor film, therefore a solar cell having acertain extent of flexibility can be obtained by the selection of thesupport substrate etc. For this reason, it is possible to use the samefor a window glass with solar cells on the glass surface, the roof of asolar car, etc.

Further, since a single crystal is excellent in opto-electric conversionefficiency, the amount of electric power generated per unit area isbetter than conventional amorphous silicon. In addition, this can beproduced with low energy, therefore the time for energy recovery can begreatly shortened.

According to the process for producing a thin film semiconductor of thepresent invention explained above, a thin film semiconductor having alarge surface area and excellent crystallinity can be easily andinexpensively produced. Further, according to the process for producinga solar cell of the present invention, a solar cell which has a largesurface area, excellent crystallinity, and sufficient thinness andaccordingly high efficiency can be cheaply produced. Due to thereduction of costs in this way, it is possible to shorten the time forrecovery of the energy.

What is claimed is:
 1. A method for making a thin film semi-conductorcomprising the steps of:providing a semi-conductor substrate having asurface; anodizing the semi-conductor substrate to provide a firstporous layer adjacent the surface having a first porosity; anodizing thesemi-conductor substrate to provide at least one second porous layeradjacent the first porous layer opposite the surface, each said secondporous layer having a second porosity greater than said first porosity;forming a semi-conductor film on the first porous layer; and separatingthe semi-conductor film from the semi-conductor substrate along a lineof relative weakness defined in or adjacent one of said second porouslayers.
 2. A method for making a thin film semi-conductor comprising thesteps of:providing a semi-conductor substrate having a surface;anodizing the semi-conductor substrate at a first current density toprovide a first porous layer adjacent the surface having a firstporosity; anodizing the semi-conductor substrate at a second currentdensity higher than said first current density to provide a secondporous layer adjacent the first porous layer opposite the surface, thesecond porous layer having a second porosity greater than the firstporosity; anodizing the semi-conductor substrate at a third currentdensity higher than said second current density to provide a thirdporous layer in or adjacent the second porous layer, the third porouslayer having a third porosity higher than said second porosity; formingat least one semi-conductor film on the surface and first porous layer;and separating the semi-conductor film from the semi-conductor substratealong a line of relative weakness defined in the third porous layer orat or adjacent an interface defined between said third porous layer andthe second porous layer.
 3. A method as defined in claim 2, wherein insaid anodizing steps, the semi-conductor substrate is contacted by anelectrolytic solution and exposed to a flow of current at said first,second and third current density, respectively.
 4. A method as definedin claim 3, wherein the electrolytic solution comprises hydrogenfluoride and a hydrocarbon alcohol.
 5. A method as defined in claim 3,wherein in the anodizing steps, the composition of the electrolyticsolution used in each anodizing step is the same.
 6. A method as definedin claim 3, wherein in the anodizing steps, the composition of theelectrolytic solution used in the anodizing steps varies.
 7. A method asdefined in claim 2, further comprising the step of annealing thesemi-conductor substrate in a hydrogen atmosphere after the thirdanodizing step and before the forming step.
 8. A method as defined inclaim 7, further comprising the step of oxidizing the anodized substrateafter the third anodizing step and before the hydrogen annealing step.9. A method as defined in claim 2, wherein in the forming step thesemi-conductor film is epitaxially grown.
 10. A method as defined inclaim 2, wherein the semi-conductor substrate is a single crystalsilicon substrate.
 11. A method as defined in claim 2, wherein thesemi-conductor substrate is an impurity-doped semi-conductor substrate.12. A method as defined in claim 2, wherein the semi-conductor substrateis a compound semi-conductor substrate.
 13. A method as defined in claim2, further comprising the step of attaching a support substrate to thesemi-conductor film after the forming step and before the separatingstep.
 14. A method as defined in claim 13, wherein the support substrateis a rigid substrate.
 15. A method as defined in claim 13, wherein thesupport substrate is a flexible substrate.
 16. A method as defined inclaim 13, wherein the support substrate is attached to thesemi-conductor film by adhesive bonding.
 17. A method for making a thinfilm semi-conductor comprising the steps of:providing a semi-conductorsubstrate having a surface; anodizing the semi-conductor substrate at afirst current density to provide a first porous layer adjacent thesurface having a first porosity; anodizing the semi-conductor substrateat a second current density higher than said first current density toprovide a second porous layer adjacent the first porous layer oppositethe surface, the second porous layer having a second porosity greaterthan the first porosity; anodizing the semi-conductor substrate at athird current density higher than said second current density to providea third porous layer in or adjacent the second porous layer, the thirdporous layer having a third porosity higher than said second porosity;forming at least one semi-conductor film on the surface and first porouslayer; and separating the semi-conductor film from the semi-conductorsubstrate along a line of relative weakness defined in the third porouslayer or at or adjacent an interface defined between said third porouslayer and the second porous layer, wherein in said anodizing steps, thesemi-conductor substrate is contacted by an electrolytic solution andexposed to a flow of current at said first, second and third currentdensity, respectively, and wherein in the anodizing steps, theelectrolytic solution is the same.
 18. A method for making a thin filmsemi-conductor comprising the steps of:providing a semi-conductorsubstrate having a surface; anodizing the semi-conductor substrate at afirst current density to provide a first porous layer adjacent thesurface having a first porosity; anodizing the semi-conductor substrateat a second current density higher than said first current density toprovide a second porous layer adjacent the first porous layer oppositethe surface, the second porous layer having a second porosity greaterthan the first porosity; anodizing the semi-conductor substrate at athird current density higher than said second current density to providea third porous layer in or adjacent the second porous layer, the thirdporous layer having a third porosity higher than said second porosity;forming at least one semi-conductor film on the surface and first porouslayer; and separating the semi-conductor film from the semi-conductorsubstrate along a line of relative weakness defined in the third porouslayer or at or adjacent an interface defined between said third porouslayer and the second porous layer, wherein in said anodizing steps, thesemi-conductor substrate is contacted by an electrolytic solution andexposed to a flow of current at said first, second and third currentdensity, respectively, and wherein the electrolytic solution used in theanodizing steps varies.
 19. A method for making a thin filnsemi-conductor comprising the steps of:providing a semi-conductorsubstrate having a surface; anodizing said semi-conductor substrate at afirst current density to provide a first porous layer adjacent saidsurface having a first porosity; anodizing said semi-conductor substrateat a second current density higher than said first current density toprovide a second porous layer adjacent said first porous layer oppositesaid surface, said second porous layer having a second porosity greaterthan said first porosity; annealing said semi-conductor substrate in ahydrogen atmosphere after said step of anodizing said semi-conductorsubstrate to provide said second porous layer; and forming at least onesemi-conductor film on said surface.
 20. A method for making a thin filmsemi-conductor comprising the steps of:providing a semi-conductorsubstrate having a surface; forming a first porous layer adjacent saidsurface having a first porosity; forming a second porous layer withinsaid first porous layer having a second porosity higher than said firstporosity; forming at least one semi-conductor film on said surface; andseparating said semi-conductor film from said semi-conductor substrate.